Fix warnings in libextra tests

Most of these are "unneccesary allocation" in bitv, for ~[false, ..] instead
of [false, ..].
Fix warnings in libstd and librusti tests
auto merge of #7857 : blake2-ppc/rust/fix-test-warnings, r=alexcrichton

Fix warnings that only show up when compiling the tests for libstd, libextra and one in librusti. Only trivial changes.