Documentation request: document all automatic variable replacements available

[hagertnl]

The harness has many automatic replacements that aren't derived from rgt_test_input.ini. For example - working_dir, scripts_dir, results_dir.
I think we need to document all the possibilities, because I am even surprised sometimes by new replacements.

[hagertnl]

In addition, we should also document all automatically-created test-specific environment variables. We have documented that the variables from machine.ini get added to the environment -- good. But we also have several variables that seem to be created by the harness automatically like

RGT_TEST_BUILD_DIR
RGT_TEST_SCRIPTS_DIR
RGT_TEST_WORK_DIR
RGT_TEST_RUNARCHIVE_DIR
RGT_TEST_STATUS_DIR
RGT_APP_SOURCE_DIR

And in our current Slurm template that we provide, we ultimately duplicate these fields by using the replacements of __working_dir__, __results_dir__, etc.

I think we might want to consider using these fields in the template and even in check scripts.
Rationale:

• I'm thinking of a case where the user runs something like "set_harness_environment.py" from within a test's Run_Archive directory, and "set_harness_environment.py" sets the RGT_TEST_* variables. Now, the user could potentially run the check/report scripts by hand (or run the check_executable_driver.py script more easily).

One pitfall that comes to mind:

• Paths wouldn't be hard-coded within the job script