Test infrastructure

Ruslan (and everybody else),

When adding some tests to the core to enable testing some link search
issues that clkao contributed a test suite for, Schwern and Autrijus
suckered me into refactoring RT’s test suite to use “prove” as a test
harness, rather than our hand-rolled harness. So now you get much nicer
test suite output and you can run many of the test files on their own.

Patches appreciated :wink:

The changes should be in 3.3.16, which I should be releasing on Tuesday.