tests/helper-runtests.sh

tests: make directory to prevent test process from going out of $TESTTMP

Before this patch, test process for test-bookmarks.t goes out of
$TESTTMP at "cd .." before creation of "orderrepo" repository.

To prevent test process from going out of $TESTTMP, this patch makes
directory "repo" sub-directory and executes almost all test scenarios
in test-bookmarks.t under it.

This is preparation for new test added in subsequent patch.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL