presubmit: check for uppercase (#1774)

Another thing we can test automatically, we sorta settled on using an
uppercase letter in in t.Log and t.Fatal calls.

Let's just check for this.

plugin/test/file_test.go

@@ -5,7 +5,7 @@ import "testing"
 func TestTempFile(t *testing.T) {
 	_, f, e := TempFile(".", "test")
 	if e != nil {
-		t.Fatalf("failed to create temp file: %s", e)
+		t.Fatalf("Failed to create temp file: %s", e)
 	}
 	defer f()
 }