Cleanup concurrency tests

In this commit I:
- removed unneeded heap allocations
- added extra whitespace to crowded expressions
- and removed unneeded syntax
auto merge of #8905 : sstewartgallus/rust/cleanup_tests, r=bblum

In this commit I:
- removed unneeded heap allocations
- added extra whitespace to crowded expressions
- and removed unneeded syntax

Also, CC @bblum although this change is fairly unobjectionable.