test(contrib/drivers/gaussdb): add soft time, with, scanlist test coverage (gogf#4686)

## Summary
- Port 3 test files and 4 testdata SQL files from PgSQL driver to
GaussDB driver
- Add `gaussdb_z_unit_feature_soft_time_test.go` (15 tests): soft time
create/update/delete, bool/int/datetime soft delete
- Add `gaussdb_z_unit_feature_with_test.go` (6 tests): With/WithAll ORM
relation queries, multiple dependency levels
- Add `gaussdb_z_unit_feature_scanlist_test.go` (9 tests): ScanList for
1:1, 1:N, N:N relation mapping
- Add 4 testdata SQL files for With relation tests
- **30 new test functions**, ~3,941 net new lines

## Test plan
- [x] `go build ./...` passes
- [x] `gofmt` and `gci` applied
- [x] No remaining `pgsql` references in new files
- [ ] Run full test suite against GaussDB instance

ref gogf#4689

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Co-authored-by: John Guo <claymore1986@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people