test: consolidate port allocation into shared test/testutil package (#8982)

* test: consolidate port allocation into shared test/testutil package

Move duplicated port allocation logic from 15+ test files into a single
shared package at test/testutil/. This fixes a port collision bug where
independently allocated ports could overlap via the gRPC offset
(port+10000), causing weed mini to reject the configuration.

The shared package provides:
- AllocatePorts: atomic allocation of N unique ports
- AllocateMiniPorts/MustFreeMiniPorts: gRPC-offset-aware allocation
  that prevents port A+10000 == port B collisions
- WaitForPort, WaitForService, FindBindIP, WriteIAMConfig, HasDocker

* test: address review feedback and fix FUSE build

- Revert fuse_integration change: it has its own go.mod and cannot
  import the shared testutil package
- AllocateMiniPorts: hold all listeners open until the entire batch is
  allocated, preventing race conditions where other processes steal ports
- HasDocker: add 5s context timeout to avoid hanging on stalled Docker
- WaitForService: only treat 2xx HTTP status codes as ready

* test: use global rand in AllocateMiniPorts for better seeding

Go 1.20+ auto-seeds the global rand generator. Using it avoids
identical sequences when multiple tests call at the same nanosecond.

* test: revert WaitForService status code check

S3 endpoints return non-2xx (e.g. 403) on bare GET requests, so
requiring 2xx caused the S3 integration test to time out. Any HTTP
response is sufficient proof that the service is running.

* test: fix gofmt formatting in s3tables test files

test/fuse_integration/framework_test.go

@@ -82,12 +82,13 @@ func NewFuseTestFramework(t *testing.T, config *TestConfig) *FuseTestFramework {
 	}
 }
 
-// freePort asks the OS for a free TCP port.
+// freePort asks the OS for a free TCP port in a range where the gRPC
+// offset (port + 10000) won't collide with well-known ports.
 func freePort(t *testing.T) int {
 	t.Helper()
 	const (
 		minServicePort = 20000
-		maxServicePort = 55535 // SeaweedFS gRPC service uses httpPort + 10000.
+		maxServicePort = 55535
 	)
 
 	portCount := maxServicePort - minServicePort + 1