runtime: skip gdb tests on Plan 9

There's no gdb on Plan 9.

Change-Id: Ibeb0fbd3c096a69181c19b1fb2bc6291612b6da3
Reviewed-on: https://go-review.googlesource.com/c/go/+/226657
Reviewed-by: David du Colombier <0intro@gmail.com>
diff --git a/src/runtime/runtime-gdb_test.go b/src/runtime/runtime-gdb_test.go
index 79b4621..5dbe4bf 100644
--- a/src/runtime/runtime-gdb_test.go
+++ b/src/runtime/runtime-gdb_test.go
@@ -41,6 +41,8 @@
 		if testing.Short() {
 			t.Skip("skipping gdb tests on AIX; see https://golang.org/issue/35710")
 		}
+	case "plan9":
+		t.Skip("there is no gdb on Plan 9")
 	}
 	if final := os.Getenv("GOROOT_FINAL"); final != "" && runtime.GOROOT() != final {
 		t.Skip("gdb test can fail with GOROOT_FINAL pending")