tests: fix an old botched change to an aclocal test

tests: fix an old botched change to an aclocal test

* t/acloca10.sh (configure.ac): Here, invoke the m4 macro 'MACRO2'
before the macro 'MACRO1' (the related test 't/aclocal-I-order-2.sh'
does the opposite).  This reverts a botched edit done (by myself,
oops) in commit 'v1.11-1335-gefdc3e1' of 2011-09-11, "tests: minor
optimizations/simplifications in some aclocal tests", and makes the
behaviour of the test match once again what is stated in the
heading comments.  While at it, improve those same heading comments
a little.

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