crypto/x509: make TestIssue51759 builder only

Since we can't gate tests on the macOS version on normal machines,
restrict TestIssue51759 to only run on builders, where we have a way to
do this.

Change-Id: I70fc83c587689b499b6a38864973a77bb3e52596
Reviewed-on: https://go-review.googlesource.com/c/go/+/472619
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: rolandshoemaker

src/crypto/x509/verify_test.go

@@ -1860,7 +1860,11 @@ func TestIssue51759(t *testing.T) {
 	if runtime.GOOS != "darwin" {
 		t.Skip("only affects darwin")
 	}
-	if builder := testenv.Builder(); builder == "darwin-amd64-10_14" || builder == "darwin-amd64-10_15" {
+	builder := testenv.Builder()
+	if builder == "" {
+		t.Skip("only run this test on the builders, as we have no reasonable way to gate tests on macOS versions elsewhere")
+	}
+	if builder == "darwin-amd64-10_14" || builder == "darwin-amd64-10_15" {
 		t.Skip("behavior only enforced in macOS 11 and after")
 	}
 	// badCertData contains a cert that we parse as valid