tests: tweak tests on silent-rules for makes without nested vars

tests: tweak tests on silent-rules for makes without nested vars

* tests/silent-nested-vars.test: Define $MAKE to `./mymake' in
the environment, so that it will be automatically picked up by
configure.  Related tweaks and simplifications.
Remove the checks verifying that silent rules are respected also
when the 'subdir-objects' option is set, the checks testing the
old syntax for user-defined silent rules, and the tests using
the default $MAKE programs: they are redundant w.r.t. other test
cases.  Related simplifications.
Remove the temporary files used in a sanity check as soon as they
are not needed anymore.
Do not capture also the standard error of configure: we only want
to grep its standard output.
Consistently use `stdout' for the name of files where to save the
standard output captured from make and configure, for consistency
with other tests.
Make grepping of configure output and of the generated Makefile
stricter.
Improve and tweak the `mymake' script a little.
* tests/silent6.test: Make grepping of make stdout slightly
stricter.  After having configured with silent rules disabled,
try to force the use silent rules with `make V=1'.