test: leave T\d\d\d- in variable $this_test and introduce $this_test_bare

test: leave T\d\d\d- in variable $this_test and introduce $this_test_bare

Script `notmuch-test` expects the results file have T\d\d\d- part
intact so the results files (and some test output files) are now
name as such.
Without this change `notmuch-test` will exit in case the test
script it was executing exited with nonzero value.

The T\d\d\d- part is dropped in new variable $this_test_bare which is
used in progress informational messages and when loading .el files in
emacs tests (whenever $this_test_bare.el exists).