tests: some improvements to Gettext tests

tests: some improvements to Gettext tests

Mostly to bring them more in sync with the ones in Automake-NG.
See also commit v1.12.2-824-g5468d52 of 2012-08-10([ng] tests:
reorganize gettext tests a bit) in Automake-NG.

* t/gettext.sh: Rename ...
* t/gettext-basics.sh: ... like this, enhance a little, and
move checks on requirement of 'config.rpath' out into ...
* t/gettext-config-rpath.sh: ... into this new test, and move
checks about PR/381...
* t/gettext-pr381.sh: ... into this new test.
* t/gettext2.sh: Rename ...
* t/gettext-external-pr338.sh: ... like this, and enhance a
little.
* t/gettext3.sh: Rename ...
* t/gettext-intl-subdir.sh: ... like this, and add trailing
':' command.

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