Move virtualfile-related tests into a separate test file

[seisman]

Description of proposed changes

• Move virtualfile-related tests into a separate test file
• Remove an invalid pylint directive

Address #2729 (comment).

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

[seisman]

Please note that the same code is also defined in test_clib.py.

[weiji14]

Hmm, the typical way of sharing fixtures in pytest would be to put them in conftest.py, but not sure about this contextmanager... We could also just keep that single test_virtual_file_fails test in test_clib.py to avoid duplicating this contextmanager?

[seisman]

I just realized the mock function is also used in other files like test_clib_put_matrix.py by from pygmt.tests.test_clib import mock.

This may not be a best practice but it works. Done in d5ef26e.

[weiji14]

Hmm, the typical way of sharing fixtures in pytest would be to put them in conftest.py, but not sure about this contextmanager... We could also just keep that single test_virtual_file_fails test in test_clib.py to avoid duplicating this contextmanager?