tests: vcs: remove influence of user's hg settings

Certain settings in the user's hgrc file can cause test failures. For
example, enabling a non-existent extension prints stderr messages like

*** failed to import extension journal: No module named journal

Tests that check stderr output for emptiness, like
test_clone_hg_repo_by_admin, then fail.

Instead, avoid all influence of the user's settings, by setting HGRCPATH and
HGPLAIN to empty.

In versions of Mercurial before 4.2, setting an empty HGRCPATH was generally
enough to get expected behavior, but since 4.2 the behavior of some commands
has changed, and a pager is now default. Setting HGPLAIN is a simple way of
neutralizing even that.