CI: Pass work directory to runTestsuite

The testsuite can now use a user-specified work directory for
all it's temp files.  This allows the docker containers to use
a tmpfs backed directory for the temp files instead of it's
own write-layer image.

* runTestsuite.sh now accepts a --work-dir command line argument
  that gets exported as AST_WORK_DIR before running the testsuite.

* gates.jenkinsfile now specifies --work-dir to be
  <testsuite_dir>/astroot.

Since the Asterisk CI docker hosts now mount /srv/jenkins/workspace
on a tmpfs, asterisk should be compiled and the testsuite run all in
memory.

Change-Id: If5ee905a15821296c355bb84cda38950ad8edc45
(cherry picked from commit a335f4c9)