bash: append srcdir to run-ptest script

bash: append srcdir to run-ptest script

If srcdir is not set, we get below error while running ptest for bash
on qemu target,

-- snip --
root@qemux86:/usr/lib/bash/ptest# ./run-ptest
/bin/sh: line 0: cd: /home/ajay/Downloads/poky-fido-13.0.0/build/tmp/work/i586-poky-linux/bash/4.3-r1/bash-4.3/tests: No such file or directory
Makefile:879: recipe for target 'runtest' failed
make: *** [runtest] Error 1
root@qemux86:/usr/lib/bash/ptest# vi run-ptest
root@qemux86:/usr/lib/bash/ptest#
-- CUT --

So, set srcdir to current directory, where tests binaries exist.

[YOCTO #8145]

Signed-off-by: Ajay M <ajay.gju@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>