gopls/internal/regtest: unskip TestSumUpdateFixesDiagnostics

Metadata reloading has been significantly refactored recently. Unskip
this test to see if it still flakes.

For golang/go#51352
For golang/go#53878

Change-Id: I9f2ae1835bbace1b5095c2d45db082c4e709437b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/423974
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Peter Weinberger <pjw@google.com>

gopls/internal/regtest/modfile/modfile_test.go

@@ -877,8 +877,6 @@ 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 = `