Inconsistencies in the pg_regress path

[daamien]

Hi !

There's an inconsistency in the pg_regress path:

• cargo pgrx new will create a folder ./pg_regress/
  https://github.com/pgcentralfoundation/pgrx/blob/develop/cargo-pgrx/src/command/new.rs#L70

• cargo pgrx regress will look for a folder ./tests/pg_regress/
  https://github.com/pgcentralfoundation/pgrx/blob/develop/cargo-pgrx/src/command/regress.rs#L648

The cargo-pgrx README adds to the confusion by talking of ./tests/pg_regress and ./pg_regress/ at the same time

https://github.com/pgcentralfoundation/pgrx/blob/develop/cargo-pgrx/README.md?plain=1#L564

https://github.com/pgcentralfoundation/pgrx/blob/develop/cargo-pgrx/README.md?plain=1#L593

As a result, right after a cargo pgrx new the command cargo pgrx regress fails with

Error:
   0: No such file or directory (os error 2

And the whole experience is kind of painfull :)

I'm willing to fix this, but I need which one of both path is the correct one.

I personnally prefer ./tests/pg_regress/ but that's just me :)