crux-mir: Restore --no-model-internal-atomics

[RyanGlScott]

#730 originally added a flag to crux-mir's translate_libs.sh script to build it in a way such that different thread interleavings are not modeled:

crucible/crux-mir/Concurrency.md

Lines 15 to 22 in a8839cd

	Some library types, such as `std::sync::Arc`, use atomic primitives internally.
	Naively exploring all thread interleavings due to executing these atomic load
	and store instructions (currently) introduces a significiant performance
	penalty, as the number of thread interleavings explodes. To avoid exploring
	these interleavings (which should generally be unobservable), we can generate a
	version of the libraries where these particular atomics are not modeled via:
	$ ./translate_libs.sh --no-model-internal-atomics

Unfortunately, I overlooked the existence of this --no-model-internal-atomics flag when I worked on #1096, and as such, #1096 accidentally removed the flag altogether. What's more, I was not aware of this because the concurrency-related parts of crux-mir are not extensively tested in the CI. We should:

1. Restore the flag, and
2. Add some test cases that demonstrate it.