TESTrun: add definitions of true and false.

TESTrun: add definitions of true and false.

If we don't give them definitions, in some contents they are interpreted
as strings, and 'false', being a non-empty string, is interpreted as
being true.  See, for example:

https://stackoverflow.com/questions/6936194/is-there-a-built-in-true-false-boolean-value-in-perl