test: Residual de-muting of the test suite. rs=demuting
test: De-~mut the test suite. rs=demuting
Revert "test: De-~mut the test suite. rs=demuting"

This reverts commit f63efdc2100ff28e2a42600641835e7bd8bde591.
librustc: Implement a lint mode for mutable structures; deny by default. r=tjc