tests: Initial version of integration tests (#7253)

### Describe Your Changes

Related issue: #7199

This is the initial version of the integration tests for cluster. See
`README.md` for details.

Currently cluster only, but it can also be used for vm-single if needed.

The code has been added to the apptest package that resides in the root
directory of the VM codebase. This is done to exclude the integration
tests from regular testing build targets because:

- Most of the test variants do not apply to integration testing (such as
pure or race).
- The integtation tests may also be slow because each test must wait for
2 seconds so vmstorage flushes pending content). It may be okay when
there are a few tests but when there is a 100 of them running tests will
require much more time which will affect the developer wait time and CI
workflows.
- Finally, the integration tests may be flaky especially short term.

An alternative approach would be placing apptest under app package and
exclude apptest from packages under test, but that is not trivial.

The integration tests rely on retrieving some application runtime info
from the application logs, namely the application's host:port. Therefore
some changes to lib/httpserver/httpserver.go were necessary, such as
reporting the effective host:port instead the one from the flag.

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>
(cherry picked from commit d7b3589dbd)

lib/httpserver/httpserver.go

@@ -114,12 +114,6 @@ func serve(addr string, useProxyProtocol bool, rh RequestHandler, idx int) {
 	if tlsEnable.GetOptionalArg(idx) {
 		scheme = "https"
 	}
-	hostAddr := addr
-	if strings.HasPrefix(hostAddr, ":") {
-		hostAddr = "127.0.0.1" + hostAddr
-	}
-	logger.Infof("starting server at %s://%s/", scheme, hostAddr)
-	logger.Infof("pprof handlers are exposed at %s://%s/debug/pprof/", scheme, hostAddr)
 	var tlsConfig *tls.Config
 	if tlsEnable.GetOptionalArg(idx) {
 		certFile := tlsCertFile.GetOptionalArg(idx)
@@ -134,6 +128,9 @@ func serve(addr string, useProxyProtocol bool, rh RequestHandler, idx int) {
 	ln, err := netutil.NewTCPListener(scheme, addr, useProxyProtocol, tlsConfig)
 	if err != nil {
 		logger.Fatalf("cannot start http server at %s: %s", addr, err)
+	} else {
+		logger.Infof("started server at %s://%s/", scheme, ln.Addr())
+		logger.Infof("pprof handlers are exposed at %s://%s/debug/pprof/", scheme, ln.Addr())
 	}
 	serveWithListener(addr, ln, rh)
 }