Add some pinclude tests

I figured this would be a good program to start by fixing because it's
probably the most likely to actually be used by other people and it's
simple to test.
Fix pinclude to work with files in ./

pinclude wasn't special-casing include files in the current directory
(as all my runs actually used it in the src/ directory for pconfigure
files).  I fixed the program and also added a test to check for this.