tests: more granularity in variables for tools overriding

tests: more granularity in variables for tools overriding

Among the other things, this change makes it possible to use, at
the same time, an autoconf version in the automake testsuite and
another one in the automake build system; even more importantly, it
makes it possible to use, say, a non-GNU make implementation in the
test scripts, while at the same time using GNU make to drive the
execution of the automake's test suite.

* tests/defs-static.in (TEX, EGREP, FGREP, SHELL, PERL, MAKE,
AUTOCONF, AUTOM4TE, AUTORECONF, AUTOHEADER, AUTOUPDATE,
original_AUTOMAKE, original_ACLOCAL): Allow and prefer overriding
through corresponding `AM_TESTSUITE_..' variables; for example,
$GREP can now be overridden by an exported `$AM_TESTSUITE_GREP'