misc/cgo/testcshared: don't run TestGo2C2Go on Darwin

Darwin doesn't support the multiple copies of the runtime package
implied by linking a c-shared library into a Go program.

Updates #29061

Change-Id: I6cf5d00babf82f1de05689c1345aaa5ae0b0659c
Reviewed-on: https://go-review.googlesource.com/c/152159
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testcshared/cshared_test.go

@@ -604,6 +604,12 @@ func copyFile(t *testing.T, dst, src string) {
 }
 
 func TestGo2C2Go(t *testing.T) {
+	if GOOS == "darwin" {
+		// Darwin shared libraries don't support the multiple
+		// copies of the runtime package implied by this test.
+		t.Skip("linking c-shared into Go programs not supported on Darwin; issue 29061")
+	}
+
 	t.Parallel()
 
 	tmpdir, err := ioutil.TempDir("", "cshared-TestGo2C2Go")