depcomp tests: fix up today's botched commit

depcomp tests: fix up today's botched commit

Today's commit 'v1.11-1959-g0699a84' has caused the generated
'depcomp*.tap' tests to call the 'plan_' function *before* it is
defined, and has also caused the extra checks required for better
libtool coverage to be run only when libtool is *not* in use,
rather than the other way around.  Fix this mess.

Report by Peter Rosin.

* tests/gen-testsuite-part: Include './defs' in the generated
tests, rather than doing so from 'depcomp.sh'.  Adjust the count
of TAP tests for 'depcomp-lt-auto.test' (it's 84, not 72).
* tests/depcomp.sh: Adjust accordingly, and fix inverted logic
about the decision of when to run extra checks for libtool.