gopls/internal/regtest/modfile: temporarily skip TestSumUpdateFixesDiagnostics

This test is somewhat noisy on the builders.
I'd like to skip it until someone can look into it in more depth.

For golang/go#51352.

Change-Id: I2aa5c9c279807b57872324d84c1bb585cdf34d06
Reviewed-on: https://go-review.googlesource.com/c/tools/+/398699
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/regtest/modfile/modfile_test.go

@@ -859,6 +859,8 @@ func main() {}
 }
 
 func TestSumUpdateFixesDiagnostics(t *testing.T) {
+	t.Skipf("Skipping known-flaky test; see https://go.dev/issue/51352.")
+
 	testenv.NeedsGo1Point(t, 14)
 
 	const mod = `