testsuite: improve interface for giving early TAP plan

testsuite: improve interface for giving early TAP plan

Since commit `v1.11-976-g4740f56', an early TAP plan can also be
specified after input lines have already been seen, as long as
no one of them is a TAP result line.  We can now take advantage
of this in our own testsuite, allowing the TAP-based scripts to
specify their earlier plan after having included `./defs'.  In
particular, this offers the benefit of allowing those scripts to
issue a "skip all" plan (e.g., "1..0 # SKIP reason ...") after
the inclusion of `./defs', which wasn't possible with the older
interface (introduced in commit `v1.11-1114-g0286308').

* tests/defs: Don't try to call `plan_' if `$planned' is defined,
the calling scripts can do that directly now.
* tests/ac-output-old.tap: Use `plan_' directly instead of
setting `$planned' before including `./defs'.
* tests/automake-cmdline.tap: Likewise.
* tests/depmod.tap: Likewise.
* tests/instspc.tap: Likewise.
* tests/tap-bad-prog.tap: Likewise.
* tests/add-missing.tap: Don't set `$planned'.