cmd/go/internal/renameio: include ios in the darwin test-flake mitigation

Because the "ios" build constraint implies "darwin", it is already
included in the general "darwin" flakiness workaround in
cmd/go/internal/robustio. We just need to relax the renameio test
to avoid false-positives there.

I do not expect this change to drive the rate of false-positives down
to zero, but it should at least reduce noise on the build dashboard.

For #42066

Change-Id: Ia33dbd33295fce5b3261b4831f2807ce29b82e65
Reviewed-on: https://go-review.googlesource.com/c/go/+/263777
Trust: Bryan C. Mills <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/renameio/renameio_test.go

@@ -144,10 +144,12 @@ func TestConcurrentReadsAndWrites(t *testing.T) {
 		// As long as those are the only errors and *some* of the reads succeed, we're happy.
 		minReadSuccesses = attempts / 4
 
-	case "darwin":
-		// The filesystem on macOS 10.14 occasionally fails with "no such file or
-		// directory" errors. See https://golang.org/issue/33041. The flake rate is
-		// fairly low, so ensure that at least 75% of attempts succeed.
+	case "darwin", "ios":
+		// The filesystem on certain versions of macOS (10.14) and iOS (affected
+		// versions TBD) occasionally fail with "no such file or directory" errors.
+		// See https://golang.org/issue/33041 and https://golang.org/issue/42066.
+		// The flake rate is fairly low, so ensure that at least 75% of attempts
+		// succeed.
 		minReadSuccesses = attempts - (attempts / 4)
 	}