run-tests: rename test description dict to testdesc
Previously the word "test" was used for both a Test instance and a path or
test dict. This patch renames them so it's clear that "testdesc" is the
dict, and "test" is the instance.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL