Tests | Remove hardcoded certificates from functional tests

[edwardneal]

This is a pre-requisite for #3014. I'm splitting this prereq in half though, it's already a little unwieldy to review.

There are a variety of hardcoded certificates (most with private keys, one without) in the test code. This PR lays out some simple enough infrastructure which helps to replace them, then replaces all the references I can see in the functional tests.

Key features here:

• TDS.Servers and TDS.EndPoint have had some minor rework. I removed the hardcoded default certificate, it'll now throw an exception if one is missing and we try to use SSL connections.
• Added a CertificateFixtureBase class to the TestUtilities project. This base class provides the infrastructure needed to generate an ad-hoc certificate. It'll use the CertificateRequest class in .NET Core, and it has to run PowerShell in .NET Framework (as per the current manual tests.) It also cleans up the certificates it leaves behind.
• Added a ColumnEncryptionCertificateFixture class to TestUtilities. This is derived from the above, and it generates a set of basic certificates, adding them to the appropriate store.
• Reworked the SqlColumnEncryptionCertificateStoreProviderShould tests, removing three hardcoded certificates (with private keys.) Switched this over to the above fixture class.
• Started to work on ExceptionsCertStore to remove its references to Utility.CreateCertificate, then realised that both of its tests were special cases of the above test class and removed them. In the process, I was also able to enable the CertificateWithNoPrivateKey test on .NET Core.
• Switched ExceptionsAlgorithmErrors to the ColumnEncryptionCertificateFixture fixture class, removing the remaining reference to Utility.CreateCertificate.
• Removed Utility.CreateCertificate (which in reality, just returned the same certificate every time.)

There are other hardcoded credentials in the manual tests, but this is already a large PR at +428/-407.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.61%. Comparing base (22ac587) to head (1ff8f1c).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3034      +/-   ##
==========================================
- Coverage   72.61%   72.61%   -0.01%     
==========================================
  Files         285      285              
  Lines       59162    59162              
==========================================
- Hits        42963    42958       -5     
- Misses      16199    16204       +5     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.42% <ø> (-0.01%)	⬇️
netfx	71.02% <ø> (+0.02%)	⬆️

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