pg_regress needs the option dlpath for postgres tests to find regress.so (#6416)
pg_regress needs the option dlpath for postgres tests to find regress.so (#6416)

When you run vanilla tests in your local environment, some of the tests
tries to find path for regress.so which is not in default lib path. That
is why we need to specify regress.so path as dlpath option.

Example failure:
```
LOAD :'regresslib';
+ERROR:  could not access file "/home/aykutbozkurt/.pgenv/pgsql-15beta4/lib/regress.so": No such file or directory
```

It is actually in
`~/.pgenv/src/postgresql-15beta4/src/test/regress/regress.so` which is
found by `$regresslibdir`.
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