tests: various minor tweakings, mostly related to AM_PROG_AR

tests: various minor tweakings, mostly related to AM_PROG_AR

* tests/alloca.test: Adjust to new portability requirements due
to the new AM_PROG_AR macro.
* tests/discover.test: Likewise.
* tests/libobj3.test: Likewise.
* tests/pluseq7.test: Likewise.  Also, make grepping of automake
expected error message stricter.
* tests/stdlib.test: Likewise, and extend the test a bit.
* tests/parse.test (configure.in): Remove redundant call to
AC_PROG_RANLIB.
* tests/library2.test: Adjust to new portability requirements
due to the new AM_PROG_AR macro.  Also ...
(configure.in): ... add call to AC_PROG_CC, to ensure automake
really fails for the expected reason.