gopls/internal/regtest: unskip TestQuickFixEmptyFiles

This test ran slowly sometimes. Now that we have improved performance,
elimininated arbitrary timeouts, and improved cacheability of
computed results when running with -short, I suspect this test should no
longer flake.

If it does, we can reduce its cost in other ways rather than turning it
off entirely.

Updates golang/go#48773
Updates golang/go#53878

Change-Id: I878e78117df5a1a25f4ac5f72e02f28fc078ec73
Reviewed-on: https://go-review.googlesource.com/c/tools/+/419106
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/regtest/diagnostics/diagnostics_test.go

@@ -964,8 +964,6 @@ const C = a.A
 // This is a copy of the scenario_default/quickfix_empty_files.txt test from
 // govim. Reproduces golang/go#39646.
 func TestQuickFixEmptyFiles(t *testing.T) {
-	t.Skip("too flaky: golang/go#48773")
-
 	testenv.NeedsGo1Point(t, 15)
 
 	const mod = `