--stderr=FILE tests were added to test/help-test as it is the one
doing most global option testing. Also, it was simplest to test
this new option using `notmuch help` command.
There are currently 45 TESTS scripts. 36 of those load
test-lib.sh using '. ./test-lib.sh' and 9 '. test-lib.sh'.
In latter case test-lib.sh is first searched from directories
in PATH (posix) and then from current directory (bash feature).
Changed the 9 files to execute '. ./test-lib.sh'. The test-lib.sh
should never be loaded from directory in PATH.