Improve a test in Common.Certificates

[TimHess]

Description

Wrap RegisterChangeCallback in usings, reset changeCalled boolean between changes

Fixes #1506

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[bart-vmware]

This is still wrong. Try debugging through it:

The changeCalled variable is set to true before the files get written.

[bart-vmware]

Thanks!

[bart-vmware]

Should probably dispose X509Certificate2 instances in tests.

[bart-vmware]

Reverting PR approval due to failing tests in cibuild.

[bart-vmware]

Wouldn't it be easier to test via OptionsMonitor, such as in the test ChangeConfiguration_AppliesChanges, combined with MemoryFileProvider, such as in the test Reloads_options_on_change? Then the sleeps can be removed.