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/file/closest_test.go

@@ -8,7 +8,7 @@ import (
 func TestClosestEncloser(t *testing.T) {
 	z, err := Parse(strings.NewReader(dbMiekNL), testzone, "stdin", 0)
 	if err != nil {
-		t.Fatalf("expect no error when reading zone, got %q", err)
+		t.Fatalf("Expect no error when reading zone, got %q", err)
 	}
 
 	tests := []struct {