build: auxiliary testsuite files/scripts built by "make all"

build: auxiliary testsuite files/scripts built by "make all"

This will allow the developers to run a tests case by hand out of
a newly extracted tarball simply doing:

    $ ./configure && make
    $ ./runtest t/the-test-case.sh

while before this change one has to resort to:

    $ ./configure && make && make check TESTS=
    $ ./runtest t/the-test-case.sh

or, with some non-GNU makes, even:

    $ ./configure && make && make check TESTS= AM_MAKEFLAGS="TESTS="
    $ ./runtest t/the-test-case.sh

This come very handy sometimes, especially when doing one-shot
debugging.  Admittedly not a big deal, but one less friction in
the build system is always nice.

* Makefile.am (check_SCRIPTS, dist_check_DATA, nodist_check_DATA):
Move their content to ...
(noinst_SCRIPTS, dist_noinst_DATA, nodist_noinst_DATA): ... these
variables.
Adjust comments.

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