Always check files in generation tests (#1847)

When iterating on new language plugins, it is helpful to have the CheckFiles
validation run so that any mismatched BUILD file output can be shown.

Replacing `FailNow` with `Fail` allows for this

testtools/files.go

@@ -368,7 +368,7 @@ Run %s to update BUILD.out and expected{Stdout,Stderr,ExitCode}.txt files.
 			for _, err := range errs {
 				t.Log(err)
 			}
-			t.FailNow()
+			t.Fail()
 		}
 
 		CheckFiles(t, testdataDir, goldens)