Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of _Concurrency APIs,
instead of disabling availability checking.
In the test infrastructure, I was using the binary
comparison tool `cmp` to check if the serialization
of a module can be deserialized and reserialized to
the exact same swiftmodule file. It turns out that
bitwise equality on the emitted files is not
guaranteed between those two methods of creating
a swiftmodule file is not always guaranteed.
Thus, this commit removes the uses of cmp to check
for this fragile guarantee.
1. Removes gating on -enable-experimental-concurrency.
2. Updates eff. prop tests to remove experimental flag,
and also adjusts some tests slightly to avoid things
that are still behind that flag.
