windows: skip TestWinVerifyTrust

This test is failing (with different failure modes) on two different
builders. Adding a skip until it can be made more robust.

For golang/go#49651
For golang/go#49266
Updates golang/go#46906

Change-Id: I0fdd0e6f729c37e234b62b65abc53003eb8834f0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/364794
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Patrik Nyblom <pnyb@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Patrik Nyblom <pnyb@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

windows/syscall_windows_test.go

@@ -627,6 +627,8 @@ func TestCommandLineRecomposition(t *testing.T) {
 }
 
 func TestWinVerifyTrust(t *testing.T) {
+	t.Skip("skipping fragile test; see https://golang.org/issue/49266 and https://golang.org/issue/49651")
+
 	system32, err := windows.GetSystemDirectory()
 	if err != nil {
 		t.Errorf("unable to find system32 directory: %v", err)