meson/docs/markdown/snippets/select_run_project_tests.md

added --only test(s) option to run_project_tests.py

Individual tests or a list of tests from run_project_tests.py can be selected like:

python run_project_tests.py --only fortran

python run_project_tests.py --only fortran python3

This assists Meson development by only running the tests for the portion of Meson being worked on during local development.