It's annoying especially for tests to not be able to append to a
config file without knowing the contents (as you have to do with
TOML). Let's read all files in a directory if `$JJ_CONFIG` points to a
directory. Mercurial does that for its `$HGRCPATH` variable.
Sometimes it's useful to have an environment variable set for all
commands in a test. This patch lets you do that by adding environment
variables to the `TestEnvironment` itself. These will then be set on
all subsequent commands.
