Add a new target 'doctest' to run doctests only and simplify Makefile

[seisman]

Description of proposed changes

This is the first step to address #2286 (comment).

Changes in this PR:

• Use Makefile's Target-specific Variable Values to simplify the Makefile targets related to tests. Now we have the _runtest target to run the tests. Other targets tests, fulltests, doctest, test_no_images all depend on the _runtest target. For each target, different values for PYTEST_ARGS are passed, so that different tests are execuated.
• Add a doctest target which only runs the doctests, as suggested in Run full tests (including inline doctests) in workflows #2286 (comment)
• Only generate coverage reports for the test target

To test this PR, you can run make <target> to see which tests run.

To save your time, you can simply add --collect-only (only collect the tests and don't run them) to pytest, i.e., changing line 37 to

cd $(TESTDIR); PYGMT_USE_EXTERNAL_DISPLAY="false" pytest --collect-only $(PYTEST_ARGS) --pyargs $(PROJECT)

Here are the counts of tests when I run these targets:

• make test: 659
• make fulltest: 696
• make doctest: 68
• make test_no_images: 696

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[willschlitzer]

I think this is a good change. I haven't worked with Makefiles, but the new syntax looks correct. Everything works on my computer.

[seisman]

Ping @weiji14 do you have more comments on the changes?

[weiji14]

Was just checking this out. Looks good!