test_vector_concurrency iterator invalidation case fails occasionally

[dfields-msft]

The test_vector_concurrency test typically passes, but may occasionally encounter the following failure:

-------------------------------------------------------------------------------
multi_threaded_vector
-------------------------------------------------------------------------------
D:\a\1\s\test\test\multi_threaded_vector.cpp(455)
...............................................................................

D:\a\1\s\test\test\multi_threaded_vector.cpp(419): FAILED:
  CHECK( conditional_unbox<T>(t) == 1 )
with expansion:
  42 == 1

===============================================================================
test cases:   80 |   79 passed | 1 failed
assertions: 2864 | 2863 passed | 1 failed
===============================================================================

It's not immediately clear whether this is a problem with the product code, or the test code. This failure may have been masked for a while due to the fact that until #1011, test failures would be silently ignored and not fail the build pipeline.

@kennykerr FYI

[kennykerr]

@dunhor does that test look familiar?

[kennykerr]

I'm guessing this is a test bug but I'll leave it unless it starts making more noise.