test-dirstate: print something when the check is skipped This makes a programming error obvious in cases when it should not be skipped Differential Revision: https://phab.mercurial-scm.org/D12602

test-dirstate: fix detection of Rust environment variable The Rust path never actually worked. This change also improves clarity of the comment. The next change will ensure we print something when this check fails. Differential Revision: https://phab.mercurial-scm.org/D12601