ci: Add a hook that runs before and a hook that runs after ctest

After this change, if a template's .ci_hooks directory contains
an executable file called 'pre_ctest', that file will be sourced
(i.e. executed without a subshell) immediately before ctest is
run in the CI process.
Similarly an executable file called 'post_ctest' will be sourced
immediately after ctest.

Signed-off-by: Chris Swinchatt <christopher.swinchatt@arm.com>