test: Make plugin tests OS independent

[Hamsajj]

Description

The tests were not passing on MacOS because the expected plugin file name was hardcoded to include linux-amd, while plugin command will download the OS-specific file. This PR fixes this issue.

Also, in api_test.go client can be nil (for example if psql is not running), added a require.NotNil to fail gracefully instead of panicking.

Development Checklist

• I have added a descriptive title to this PR.
• I have squashed related commits together.
• I have rebased my branch on top of the latest main branch.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added docstring(s) to my code.
• I have made corresponding changes to the documentation (docs).
• I have added tests for my changes.
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[mostafa]

Hey @Hamsajj,

Thanks a lot for your contribution! 🙏