Summary:
The test suite is unrelated to PgTxnTest and should be placed in its own file.

To review the change, it is recommended to

```
git show HEAD~:src/yb/yql/pgwrapper/pg_txn-test.cc | sed -n '422,929p' > ../old.cc
git diff ../old.cc HEAD:src/yb/yql/pgwrapper/pg_read_after_commit_visibility-test.cc
```
Jira: DB-14697

Test Plan:
Jenkins

```
ybd --cxx-test pg_read_after_commit_visibility-test
ybd --cxx-test pg_txn-test
```

Reviewers: pjain, smishra

Reviewed By: pjain

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D42405