If symbol-test is built in symbol-hiding with hardcoded g++ invokation,
it's not so easy to pass $(srcdir) which is required to find notmuch.h
when srcdir and builddir are separate directories.
Change add_email_corpus, emacs_deliver_message and tests to use
$TEST_DIRECTORY instead of '..'.
This improves the behavior of the usage of --root=<dir>, as the
assumption of what '..' means will usually be incorrect.
Document -root option in README and update valgrind to work with
-root.
The worry here is that a binary linking with libnotmuch might lose
access to Xapian::Error symbols because libnotmuch hides them.
We are careful here to create ./fakedb/.notmuch in order to trigger a
Xapian exception, and not just a missing file check.
Thanks to jrollins and amddragon for suggestions.
(cherry picked from commit 66f37f5f6864a988f94ddb893e3a176af57f6c8e)
