tests: Add context cancelation (#15374)

A huge number of goroutines would build up from various monitors

When creating test filesystems provide a context so they can shut down when no longer needed.

cmd/signature-v4_test.go

@@ -18,6 +18,7 @@
 package cmd
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -93,7 +94,10 @@ func TestDoesPolicySignatureMatch(t *testing.T) {
 }
 
 func TestDoesPresignedSignatureMatch(t *testing.T) {
-	obj, fsDir, err := prepareFS()
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	obj, fsDir, err := prepareFS(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}