lib/fasttime: remove performance overhead when UnixTimestamp() is called from benchmark loops

Move the synctest-related implementation into a separate file protected with go:build goexperiment.synctest.

This is a follow-up for the commit 06c26315df
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/8740

lib/fasttime/fasttime.go

@@ -1,42 +1,5 @@
 package fasttime
 
-import (
-	"testing"
-	"time"
-
-	"github.com/VictoriaMetrics/VictoriaMetrics/lib/atomicutil"
-)
-
-func init() {
-	go func() {
-		ticker := time.NewTicker(time.Second)
-		defer ticker.Stop()
-		for tm := range ticker.C {
-			t := uint64(tm.Unix())
-			currentTimestamp.Store(t)
-		}
-	}()
-}
-
-var currentTimestamp = func() *atomicutil.Uint64 {
-	var x atomicutil.Uint64
-	x.Store(uint64(time.Now().Unix()))
-	return &x
-}()
-
-// UnixTimestamp returns the current unix timestamp in seconds.
-//
-// It is faster than time.Now().Unix()
-func UnixTimestamp() uint64 {
-	if testing.Testing() {
-		// When executing inside the tests, use the time package directly.
-		// This allows to override time using synctest package.
-		return uint64(time.Now().Unix())
-	}
-
-	return currentTimestamp.Load()
-}
-
 // UnixDate returns date from the current unix timestamp.
 //
 // The date is calculated by dividing unix timestamp by (24*3600)

lib/fasttime/fasttime_normal.go

@@ -0,0 +1,33 @@
+//go:build !goexperiment.synctest
+
+package fasttime
+
+import (
+	"time"
+
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/atomicutil"
+)
+
+func init() {
+	go func() {
+		ticker := time.NewTicker(time.Second)
+		defer ticker.Stop()
+		for tm := range ticker.C {
+			t := uint64(tm.Unix())
+			currentTimestamp.Store(t)
+		}
+	}()
+}
+
+var currentTimestamp = func() *atomicutil.Uint64 {
+	var x atomicutil.Uint64
+	x.Store(uint64(time.Now().Unix()))
+	return &x
+}()
+
+// UnixTimestamp returns the current unix timestamp in seconds.
+//
+// It is faster than time.Now().Unix()
+func UnixTimestamp() uint64 {
+	return currentTimestamp.Load()
+}

lib/fasttime/fasttime_synctest.go

@@ -0,0 +1,13 @@
+//go:build goexperiment.synctest
+
+package fasttime
+
+import (
+	"time"
+)
+
+// UnixTimestamp returns the current unix timestamp in seconds.
+func UnixTimestamp() uint64 {
+	// Fall back to time.Now().Unix(), since this is needed for synctest.
+	return uint64(time.Now().Unix())
+}