selftest: pass location of perl executable from waf to test-envs

selftest: pass location of perl executable from waf to test-envs

Many perl scripts in the codebase are executables with a
"/usr/bin/perl" shebang. Running them as executables is not
portable as some OS's have a different location for the perl
interpreter.

During the configuration process, waf finds the location of the perl
interpreter. Some or all  invocations of perl scripts from within
test environment setup code are actually "$PERL <script>",
but since PERL env var is typically not set, this amounts to the
unportable "<script>", which invokes /usr/bin/perl.

This patch exports the location of perl as found by the configuration
process to the test environment, causing "$PERL <script>" to be
"<correct place of perl interpreter> <script>".

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>