test: Add skip condition for `rglplot` test

[mfansler]

The rgl package is a Suggests dependency. As such, testing of functions depending on rgl should skip if it is not installed. This PR adds such a skip condition.

Test failures due to lack of skip condition were encountered during Conda Forge builds of v2.0.3.

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[krlmlr]

Thanks!

[aviator-app[bot]]

This pull request failed to merge: PR cannot be automatically rebased, please rebase manually to continue. Remove the blocked label to re-queue.

Additional debug info: Failed to rebase this PR onto the latest changes from the base branch. You will probably need to rebase this PR manually and resolve conflicts).

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This pull request failed to merge: some CI status(es) failed. Remove the blocked label to re-queue.

Failed CI(s): Check ubuntu-20.04 (oldrel-1), Check ubuntu-20.04 (oldrel-4), Check ubuntu-20.04 (oldrel-2)