selftest: change to src dir for panic backtrace

selftest: change to src dir for panic backtrace

When running selftest against a Samba3 target, the working directory is
set to st/s3dc/share. The existing "panic action" script attempts
obtain a backtrace for a paniced smbd process using GDB, which does not
locate debug info relative to the working directory.

This commit changes the S3 selftest panic action to first enter
the base source directory before attempting to obtain the backtrace,
ensuring that GDB can locate the debug info.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Sep 12 00:19:39 CEST 2013 on sn-devel-104