presubmit: Check errorf as well (#1845)

Uppercase all these test errors as well. And extend the presubmit to
check for these in the future. Also do a slightly smarter grep to only
get t.<something>. as (because dump regexp) this also grep over non test
files.

plugin/pkg/edns/edns_test.go

@@ -12,7 +12,7 @@ func TestVersion(t *testing.T) {
 
 	_, err := Version(m)
 	if err == nil {
-		t.Errorf("expected wrong version, but got OK")
+		t.Errorf("Expected wrong version, but got OK")
 	}
 }
 
@@ -22,7 +22,7 @@ func TestVersionNoEdns(t *testing.T) {
 
 	_, err := Version(m)
 	if err != nil {
-		t.Errorf("expected no error, but got one: %s", err)
+		t.Errorf("Expected no error, but got one: %s", err)
 	}
 }