test-container: Support $(complocaledir) and mkdirp.

Expand the support infrastructure:
- Create $(complocaledir) in the testroot.pristine to support localedef.
- Add the variable $complocaledir to script support.
- Add the script command 'mkdirp'.

All localedef tests which run with default paths need to have the
$(complocaledir) created in testroot.pristine. The localedef binary
will not by itself create the default path, but it will write into
the path. By adding this we can simplify the localedef tests.

The variable $complocaledir is the value of the configured
$(complocaledir) which is the location of the compiled locales that
will be searched by the runtime by default.

The command mkdirp will be available in script setup and will
be equivalent to running `mkdir -p`.

The variable and command can be used to write more complex tests.

Reviewed-by: DJ Delorie <dj@redhat.com>