CI: Add script for running tests from github worklow

[slangeveld]

Resolves part of https://github.com/equinor/rms-sys/issues/267

Added a bash script that can be run from the github workflows to execute the xtgeo unit tests towards and RMS Python Enviroment. Note that the xtgeo-testdata files needed to run the tests are cached on the github actions runner, so pointing to the path XTGEO_TESTDATA_PATH with the cached data should be sufficient to run the tests.

The script copies the required test files, installs the test dependencies and and runs the pytests.

[mferrera]

Same as other review, approving for convenience

[slangeveld]

A few changes where made based on the discussions in #1297

• Roxar tests are included when running pytest
• The pip install arguments --upgrade --force-reinstall have been removed from the command in the script as we decided to create a _rmsenv for each of the projects we are testing.
• The xtgeo-testdata is now cached on the github runner, so cloning the xtgeo-testdata repo is no longer part of the test script.