Speed up allocation counting tests

[glbrntt]

Motivation:

The allocation counting tests take around 23 minutes in CI, which is quite a while! The tests themselves don't take nearly so long, however each test is run 11 times but between each time the framework waits for allocations to become rebalanced (i.e. everything we allocated should be freed). The framework will wait up to 5 seconds between each iteration for this. About 15 or so tests take 55 seconds, meaning they're just waiting for the allocs to quiece. It turns out that dispatch doesn't always free blocks from async work items immediately. This is relevant because a number of tests create their own MTELGs, and so when they are shutdown we see allocations from the shutdown flow lingering for more than 5 seconds.

Not only does this slow the tests down, if the allocations don't quiece, the framework ignores the result, so we lose some coverage too.

Modifiications:

• Add a pre-warmed singleton MTELG which tests can use, this ensures it has plenty of space in its queues so that intermediate allocs from running tests aren't seen as leaks
• Cancel tasks which are scheduled to run after the test to avoid 'leaks'.
• Avoid using swift run and just execute the test binary, this shaves off a few seconds for each test.
• Print out how long each test run takes to make debugging easier
• Fix a bug where the framework prints out logs from all tests when checking the results of each test

Result:

• Faster alloc tests
• 'Integration tests' CI job is ~10 minutes faster