Fix test adapter thread safety

[iMacTia]

Description

Fix test adapter thread safety.
This is a follow-up attempt from #1379
Fixes #1365

Additional Notes

Tested locally using "Steps to reproduce" from #1365.
Below are the benchmark results:

# With sleep
            user     system      total        real
Before: 3.547856   0.051316   3.599172 ( 13.638507)
After:  4.511469   0.051330   4.562799 ( 14.601563)

# Without sleep
            user     system      total        real
Before: 2.324904   0.034064   2.358968 (  2.357287)
After:  2.897183   0.030553   2.927736 (  2.925748)

The change does, as expected, slow down the test adapter, but this doesn't seem to be too much (~ 24%).
Considering this is mostly used in test environments rather than production ones, it should be safe to have a small dip in performance. It's also worth noting that this only applies to a multi-threaded environment.
Most applications will use the test adapter in a single thread and normally recreate the connection for each test example, so there would be no performance impact in that scenario

[olleolleolle]

👍

Easy to view the change with less significant formatting:
https://github.com/lostisland/faraday/pull/1380/files?w=1

[iMacTia]

Nice trick! I didn't know that 😮!

I've asked @tchutw to have a look, I 'd wait for that before merging this time 😅