Use complete configure.ac’s in testsuite.

Use complete configure.ac’s in testsuite.

Autoconf 2.70 will issue warnings if it encounters a configure.ac that doesn’t
call both AC_INIT and AC_OUTPUT.

Automake already issues warnings if it encounters a configure.ac that uses an
AM_ macro but doesn’t call AM_INIT_AUTOMAKE or AC_CONFIG_FILES([Makefile]).

In two places, the testsuite was tripping these warnings, leading to spurious
failures with Autoconf 2.70 betas.

* t/aminit-moreargs-deprecation.sh: Add AC_OUTPUT to test configure.ac.
* t/mkdirp-deprecation.sh: Use a complete test configure.ac, not a stub
  containing only a use of AM_PROG_MKDIR_P.