Makefile: fix synctest tests

The `synctest` experiment has been removed in go 1.26. This causes the test
build targets to fail. To fix that, this commit removes the
`GOEXPERIMENT=synctest` env var from the build rule recipes.

This forces the go tests to expect different synctest method names and function
signatures. Thus, the synctest tests also have been fixed.

VictoriaMetrics code uses `lib/fasttime` to get the current time. This does not
work with synctest tests since they require `time.Now()` to properly override
the current time. On the other hand, the rest of the tests (and especially
benchmarks) want to continue using `lib/fasttime`. Therefore the `synctest`
build label is used to link modified version of `lib/fasttime` when the synctest
tests need to be run.

Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>

Makefile

@@ -443,7 +443,7 @@ fmt:
 	gofmt -l -w -s ./apptest
 
 vet:
-	GOEXPERIMENT=synctest go vet ./lib/...
+	go vet -tags=synctest ./lib/...
 	go vet ./app/...
 	go vet ./apptest/...
 
@@ -452,19 +452,19 @@ check-all: fmt vet golangci-lint govulncheck
 clean-checkers: remove-golangci-lint remove-govulncheck
 
 test:
-	GOEXPERIMENT=synctest go test ./lib/... ./app/...
+	go test -tags=synctest ./lib/... ./app/...
 
 test-race:
-	GOEXPERIMENT=synctest go test -race ./lib/... ./app/...
+	go test -tags=synctest -race ./lib/... ./app/...
 
 test-pure:
-	GOEXPERIMENT=synctest CGO_ENABLED=0 go test ./lib/... ./app/...
+	CGO_ENABLED=0 go test -tags=synctest ./lib/... ./app/...
 
 test-full:
-	GOEXPERIMENT=synctest go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
+	go test -tags=synctest -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
 
 test-full-386:
-	GOEXPERIMENT=synctest GOARCH=386 go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
+	GOARCH=386 go test -tags=synctest -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
 
 integration-test:
 	$(MAKE) apptest
@@ -490,11 +490,11 @@ integration-test-legacy: victoria-metrics vmbackup vmrestore
 	go test ./apptest/tests -run="^TestLegacySingle.*"
 
 benchmark:
-	GOEXPERIMENT=synctest go test -bench=. ./lib/...
+	go test -bench=. ./lib/...
 	go test -bench=. ./app/...
 
 benchmark-pure:
-	GOEXPERIMENT=synctest CGO_ENABLED=0 go test -bench=. ./lib/...
+	CGO_ENABLED=0 go test -bench=. ./lib/...
 	CGO_ENABLED=0 go test -bench=. ./app/...
 
 vendor-update:
@@ -524,7 +524,7 @@ install-qtc:
 
 
 golangci-lint: install-golangci-lint
-	GOEXPERIMENT=synctest golangci-lint run
+	golangci-lint run --build-tags=synctest
 
 install-golangci-lint:
 	which golangci-lint && (golangci-lint --version | grep -q $(GOLANGCI_LINT_VERSION)) || curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(shell go env GOPATH)/bin v$(GOLANGCI_LINT_VERSION)

lib/fasttime/fasttime_normal.go

@@ -1,4 +1,4 @@
-//go:build !goexperiment.synctest
+//go:build !synctest
 
 package fasttime
 

lib/fasttime/fasttime_synctest.go

@@ -1,4 +1,4 @@
-//go:build goexperiment.synctest
+//go:build synctest
 
 package fasttime
 

lib/storage/search_synctest_test.go

@@ -1,4 +1,4 @@
-//go:build goexperiment.synctest
+//go:build synctest
 
 package storage
 
@@ -13,7 +13,7 @@ import (
 func TestSearch_metricNamesIndifferentIndexDBs(t *testing.T) {
 	defer testRemoveAll(t)
 
-	synctest.Run(func() {
+	synctest.Test(t, func(t *testing.T) {
 		const numSeries = 10
 		tr := TimeRange{
 			MinTimestamp: time.Now().UnixMilli(),

lib/storage/storage_synctest_test.go

@@ -1,4 +1,4 @@
-//go:build goexperiment.synctest
+//go:build synctest
 
 package storage
 
@@ -289,7 +289,7 @@ func TestStorageRotateIndexDBPrefill(t *testing.T) {
 	f := func(t *testing.T, opts OpenOptions, prefillStart time.Duration) {
 		t.Helper()
 
-		synctest.Run(func() {
+		synctest.Test(t, func(t *testing.T) {
 			// Prefill of the next partition indexDB happens during the
 			// (nextMonth-prefillStart, nextMonth] time interval.
 			// Advance current time right before the the beginning of that interval.

lib/streamaggr/streamaggr_synctest_test.go

@@ -1,4 +1,4 @@
-//go:build goexperiment.synctest
+//go:build synctest
 
 package streamaggr
 
@@ -16,7 +16,7 @@ import (
 func TestAggregatorsSuccess(t *testing.T) {
 	f := func(inputMetrics []string, interval time.Duration, outputMetricsExpected, config, matchIdxsStrExpected string) {
 		t.Helper()
-		synctest.Run(func() {
+		synctest.Test(t, func(t *testing.T) {
 			var matchIdxs []uint32
 			var tssOutput []prompb.TimeSeries
 			var tssOutputLock sync.Mutex

lib/workingsetcache/cache_synctest_test.go

@@ -1,4 +1,4 @@
-//go:build goexperiment.synctest
+//go:build synctest
 
 package workingsetcache