test: misc. test cleanup

[hawkw]

this branch makes the following minor test tweaks:

• use /usr/bin/env bash in loom scripts
• fix issues with thread ID reuse making loom unhappy
• separate loom and non-loom tests into separate cfgs
• always use loom when --cfg loom is set (so we don't make other crates' loom tests nondeterministic by sharing TIDs globally)
• don't run doctests in the loom CI job

[matklad]

I wonder if loom dependency can be made into a dev dependnecy or an optional feature-gated dependency?

Although it is not being build, it does end up in downstream Cargo.locks: https://github.com/rust-analyzer/rust-analyzer/pull/6344/files#diff-13ee4b2252c9e516a0547f2891aa2105c3ca71c6d7a1e682c69be97998dfc87eR846-R858