tests: merge, tweak and modernize few test scripts

tests: merge, tweak and modernize few test scripts

Basically an adjusted-and-improved cherry-pick from Automake-NG
commit v1.12.1-343-gff30f83.

* t/specflg.sh, t/specflg2.sh, t/specflg3.sh: Merged into ...
* t/per-target-flags.sh: ... this test.
* t/fo.sh: Remove, its weak grepping checks well superseded by
the semantic checks in 't/fort4.sh'.
* t/cxxo.sh: Remove, its weak grepping checks well superseded
by the semantic checks in 't/cxx-demo.sh'.
* t/cxxcpp.sh: Enhance a little.
* t/empty.sh: Renamed ...
* t/empty-data-primary.sh: ... to this.  Add trailing ':' command.
* t/empty2.sh, t/empty3.sh, t/empty4.sh: Merged ...
* t/empty-sources-primary.tap: ... into this new test.
* t/no-outdir-option.sh: Remove.  A test to check than an obsolete
and now deleted option ("--output-dir") stays deleted is way too
much even for the most test-infected person ;-)
* t/list-of-tests.mk: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>