TestPumaServerSSLClient - share Context [changelog skip]

[MSP-Greg]

Description

While working in my fork, JRuby would intermittently have failures/timeouts from the tests in TestPumaServerSSLClient.

These tests can share the SSL Context, so adjust code to create a shared one. Note that the tests in TestPumaServerSSL yield the Context, so sharing them won't work.

Note that CI passed in my fork:
https://github.com/MSP-Greg/puma/runs/1109220969

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added an entry to History.md if this PR fixes a bug or adds a feature. If it doesn't need an entry to HISTORY.md, I have added [changelog skip] or [ci skip] to the pull request title.
• I have added appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.