test: aggregate-results updates

notmuch-test will now call aggregate-results.sh with file list
that it compiles based on the test ran, and aggregate-results
will report failure is any of the test files are missing.

With this notmuch-test no longer has to exit in non-parallel
run if some test fail to write its report file -- so it works
as parallel tests in this sense.

Changed test_done() in test-lib.sh write report file in one write(2),
so there is (even) less chance it being partially written. Also,
now it writes 'total' last and aggregate-results.sh expects this
line to exist in all report files for reporting to be successful.

Added 'set -eu' to notmuch-test and modified code to work with
these settings. That makes it harder to get mistakes slipped
into committed code.