Migrate test cases with verbose stderr logging to env_logger

[zacchiro]

Tests concerned: test_bvcomp, test_par_bvcomp, transpose

Rationale: all these tests had very verbose logging (e.g., what each thread is doing) on stderr by default, completely dominating the amount of test output. With this change they log to the env_logger instead, which by default is silent. In case of test failure, the test can be rerun with output with something like:

RUST_LOG=info cargo t -- --nocapture

[zacchiro]

Note: this is suboptimal when the test fails, as one has to manually rerun the failing tests with specific options to see debug output. But I have not found a way to make that happen by default, i.e.: log nothing if the test goes well, log something when it fails. Improvements on this point welcome!