runtime: skip gdb tests earlier before blocking goroutines in a t.Parallel

Minor.

Makes reading failing runtime test stacktraces easier (by having fewer
goroutines to read) on machines where these gdb tests wouldn't have
ever run anyway.

Change-Id: I3fab0667e017f20ef3bf96a8cc4cfcc614d25b5c
Reviewed-on: https://go-review.googlesource.com/81575Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime-gdb_test.go

@@ -110,8 +110,8 @@ func testGdbPython(t *testing.T, cgo bool) {
 		t.Skip("skipping because cgo is not enabled")
 	}
 
-	t.Parallel()
 	checkGdbEnvironment(t)
+	t.Parallel()
 	checkGdbVersion(t)
 	checkGdbPython(t)
 
@@ -266,8 +266,8 @@ func TestGdbBacktrace(t *testing.T) {
 		testenv.SkipFlaky(t, 15603)
 	}
 
-	t.Parallel()
 	checkGdbEnvironment(t)
+	t.Parallel()
 	checkGdbVersion(t)
 
 	dir, err := ioutil.TempDir("", "go-build")
@@ -336,8 +336,8 @@ func main() {
 // TestGdbAutotmpTypes ensures that types of autotmp variables appear in .debug_info
 // See bug #17830.
 func TestGdbAutotmpTypes(t *testing.T) {
-	t.Parallel()
 	checkGdbEnvironment(t)
+	t.Parallel()
 	checkGdbVersion(t)
 
 	dir, err := ioutil.TempDir("", "go-build")
@@ -401,8 +401,8 @@ func main() {
 `
 
 func TestGdbConst(t *testing.T) {
-	t.Parallel()
 	checkGdbEnvironment(t)
+	t.Parallel()
 	checkGdbVersion(t)
 
 	dir, err := ioutil.TempDir("", "go-build")