Add end-to-end tests for timestamp insertion, and patch the conversion issue

[ryanpbrewster]

Summary

This PR introduces an e2etests/ directory with "end to end" tests:

• set up a memory engine
• start the server
• connect using the normal Go mysql driver
• execute queries

This can catch issues that a unit test against the engine itself cannot.

There is only a single test thus far, and it is fairly verbose. I know that there is at least one more issue with timestamps, and I will open a follow-up PR to address that issue as soon as I track down its source. For now I'm opting for clarity rather than code reusability.

Lastly, this PR introduces a little patch to the timestamp conversion logic, which was accidentally broken in #1061 because it only handles string but the server layer now passes it []byte. This patch is very dumb and simple: just convert []byte to string before parsing.

Motivation

#1139

[ryanpbrewster]

@fulghum can you recommend a reviewer for this PR?

[fulghum]

Looks great @ryanpbrewster. Thank you for taking the time to contribute this fix!

You're right that we don't have a good end-to-end testing framework, and it's not too uncommon to see issues that only manifest when sending the request/response over the wire, so your new e2e test looks very reasonable to me.

I just had two really trivial suggestions. I think I can go ahead apply those for you and then merge this fix in, otherwise if you can accept those suggestions, I'll get 'em merged in.

[fulghum]

Code looks good and CI tests are all passing now.

Thank you for taking the time to contribute this fix to our repo! 💙