tests: test mdate-sh with /bin/sh too

tests: test mdate-sh with /bin/sh too

* tests/mdate5.test: Fetch the `mdate-sh' script using the
`get_shell_script' function, and run it directly instead of
using `$SHELL'.
* tests/mdate6.test: Likewise.  Since we are at it, make checks
on the `mdate-sh' output stricter, remove now unneeded calls to
aclocal and automake and creation/extension of `configure.in',
`Makefile.am' and `textutils.tex' files, and add a trailing `:'
command.
* tests/Makefile.am (config_shell_tests): Add `mdate5-w.shtst'
and `mdate6-w.shtst'.