Update the test_subplot_outside_plotting_positioning test and increase the RMS tolerance

[seisman]

Description of proposed changes

The test test_subplot_outside_plotting_positioning fails in the latest run of the "GMT Legacy Tests" (see https://github.com/GenericMappingTools/pygmt/actions/runs/4474241635/jobs/7862497307).

It fails due to an upstream bug that affects GMT 6.3.0 only (see GenericMappingTools/gmt#6100).

In this PR, I remove frame=True from the test so that it works with both GMT 6.3 and 6.4.

I also increase the RMS tolerance to 4.0 so that the test passes for GMT master branch.

Fixes #

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]

/test-gmt-dev

Workflow run at https://github.com/GenericMappingTools/pygmt/actions/runs/4497014198.

[seisman]

Now the test passes in the Tests (https://github.com/GenericMappingTools/pygmt/actions/runs/4497107567?pr=2454), the "GMT Dev Tests" (https://github.com/GenericMappingTools/pygmt/actions/runs/4497014198) and also the "GMT Legacy Tests" (https://github.com/GenericMappingTools/pygmt/actions/runs/4497107573).

The two failures in the "GMT Legacy Tests" are related to the doctests, which will be addressed in #2286, #2443 and #2450.