Implement "SIMA_TESTDIR" variable for use in specifying regression test paths

### What is the feature/what would you like to discuss?

Currently all physics snapshot regression tests have hardcoded paths to the `ncdata` and `ncdata_check` files used in the test themselves. This might become a problem in the future when CAM-SIMA is ported to a new machine with a different file path, as it will require the modification of many different testing files.

To alleviate this problem, a new `SIMA_TESTDIR` CIME variable should be created that contains the initial, machine-dependent part of the path. That way when CAM-SIMA is ported it will just take a single modification in a single location for a user to properly set the paths for all physics snapshot files, or any other future tests that use a modified `ncdata` file.

### Is there anyone in particular you want to be part of this conversation?

@cacraigucar @jimmielin

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Yes

Start by locating the physics snapshot regression tests and the CIME configuration where machine-dependent test paths are defined. Trace how the ncdata and ncdata_check paths are currently specified, then verify that a single SIMA_TESTDIR setting supplies their shared initial path without changing regression-test answers.

Written by the indexing model from the issue text.