Add function to locate the absolute path of a test plan

Add a function 'roundup_pwd()' to allow from inside a test plan to
locate it's current absolute position in the file system.

Using this function one can then locate other files located relative
to the plan file, e.g.

+-----------------------------------------------
| describe "Tests using some common functions from lib"
| . $(roundup_pwd)/../lib/common.sh
| it_works()
| {
| 	use_func_from_common
| }
+------------------------------------------------