Stop setting chplenv variables in start_test

[jabraham17]

This PR removes several lines from start_test.py which override certain environment variables.

Testing

• paratest with/without gasnet

[Reviewed by @]

Rationale

I had to do this, because after #26501 setting CHPL_GASNET_SEGMENT=none if CHPL_COMM=gasnet is an error. This led to confusing behavior with several tests (see #26998). It also broke test/gpu/native/environment/gasnet.chpl. chpl --comm gasnet test/gpu/native/environment/gasnet.chpl is not an error, but start_test test/gpu/native/environment/gasnet.chpl (which has a compopts with --comm gasnet fails). This is confusing, and the easiest way to fix it is to fix start_test.py.

While I only had to remove setting CHPL_GASNET_SEGMENT, I found I could remove all of these variables. They create a confusing printout that doesn't match what the runner of start_test sees.

A diff of before/after this PR. Note that now the * (meaning a user override a variable in the environment) is now actually accurate.

21c21
< CHPL_COMM: none
---
> CHPL_COMM: none *
23c23
<   CHPL_GASNET_SEGMENT: none
---
>   CHPL_GASNET_SEGMENT: none *
26,27c26,27
< CHPL_TASKS: qthreads
< CHPL_LAUNCHER: none
---
> CHPL_TASKS: qthreads *
> CHPL_LAUNCHER: none *
35c35
<   CHPL_NETWORK_ATOMICS: none
---
>   CHPL_NETWORK_ATOMICS: none *
40c40
< CHPL_LLVM: system
---
> CHPL_LLVM: system *

Searching through the git history, I think the reason that these were override in the environment is a long-time holdover from when the test scripts used to be monolithic shell scripting, rather than a separate start_test.py and sub_test.py. This was needed to make skipifs and the like work. Today, other logic handles that, making this redundant.

To fully support this, I also had to update the logic for finding good files/executing scripts to not rely on environment variables

[jabraham17]

Closing, this is an annoying legacy problem that cannot be easily solved. I've sunk too much time already into trying to make this proper fix work. I will open a separate PR that is a more targeted patch for the specific problem this is trying to solve (i.e. start_test test/gpu/native/environment/gasnet.chpl not working)