copy tests: test all supported modes

if we run the test in parallel, we always test the latest mode

see here

https://github.com/golang/go/wiki/LoopvarExperiment

also fix a lint warning about pgtype.Vec2

copy_from_test.go

@@ -17,8 +17,6 @@ import (
 func TestConnCopyWithAllQueryExecModes(t *testing.T) {
 	for _, mode := range pgxtest.AllQueryExecModes {
 		t.Run(mode.String(), func(t *testing.T) {
-			t.Parallel()
-
 			ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 			defer cancel()
 
@@ -81,8 +79,6 @@ func TestConnCopyWithKnownOIDQueryExecModes(t *testing.T) {
 
 	for _, mode := range pgxtest.KnownOIDQueryExecModes {
 		t.Run(mode.String(), func(t *testing.T) {
-			t.Parallel()
-
 			ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 			defer cancel()