tests: avoid extra test generation (for `instspc' tests)

tests: avoid extra test generation (for `instspc' tests)

We don't really need to generate tons and tons of wrapper test
scripts (presently, ~ 90) just to run what is basically a single
test (specifically, `instspc-tests.sh') with different parameters.
In fact, the possibility of running a single driver script with
multiple files/configurations/parameters is one of the major
selling points of the 'parallel-tests' testsuite driver of
automake.  So why not use ourselves more extensively the best
features we provide?

* tests/instspc-tests.sh: Update heading comments.  Expect
to be executed, not sourced.  Update handling of command-line
arguments.  Adjust to reflect the renaming of "instspc tests"
from `instspc-*.test' to `*.instspc'.
* tests/instspc-data.test: Update heading comments.  Execute
driver script `instspc-tests.sh', instead of sourcing it.
Give more informative messages in case of internal errors.
* tests/Makefile.am (TESTS_EXTENSIONS): Define explicitly (to
`.test' and `.instspc').
(TESTS): Add `$(instspc_tests)'.
(generated_tests): Remove `$(instspc_tests)'.
(INSTSPC_LOG_COMPILER): Define.  Calls `instspc-tests.sh', so
that the "instspc tests" will be now executed by passing that
driver script a proper parameter.
($(instspc_tests)): Remove, we don't need anymore to generate
this tests.
($(instspc_tests:.test=.log)): Remove, substituted by ...
($(instspc_tests:.instspc=.log)): ... this.
($(instspc_tests)): New dependency declaration (dummy, but
required in order to have make actually produce expected log
files from the `.instspc.log' suffix rule).
(MAINTAINERCLEANFILES, generated_tests): Don't extend with
$(instspc_tests) anymore.
Update comments.