Use consistent docstrings for test files

[seisman]

Description of proposed changes

Closes#2571.

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

[yvonnefroehlich]

I am wondering whether we also should update these docstrings:

$ grep "Tests" *
grep: baseline: Is a directory
grep: data: Is a directory
test_clib_loading.py:# Tests for load_libgmt
test_clib_loading.py:# Tests for clib_full_names
test_helpers.py:Tests the helper functions/classes/etc used in wrapping GMT.
test_io.py:Tests for input/output (I/O) utilities.

[seisman]

I am wondering whether we also should update these docstrings:

$ grep "Tests" *
grep: baseline: Is a directory
grep: data: Is a directory
test_clib_loading.py:# Tests for load_libgmt
test_clib_loading.py:# Tests for clib_full_names
test_helpers.py:Tests the helper functions/classes/etc used in wrapping GMT.
test_io.py:Tests for input/output (I/O) utilities.

Done in 09fb8bf.