[Tests] Use a plain array of std::atomic rather than a vector of atomic.
[Tests] Use a plain array of std::atomic rather than a vector of atomic.

The restrictions on std::atomic make a it fragile to have a vector of them, and this failed to compile on Linux in CI.

While I'm in there, clean up a couple of tests that repeated a raw `10` for the thread count.

rdar://problem/49709062
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