Inject test executable parent directory into PATH for tests

[dylanmckay]

The tests do stuff like

@cxx foo.c -o - | avr-sim

This assumes that avr-sim in in the PATH. Currently I have to cargo install avr-test-suite globally so the tests run.

avr-sim is also a part of this repository and thus it has executables generated to the same folder. If we inject the directory of the avr-lit executable when running tests into the PATH, RUN commands in tests will be able to execute avr-sim without it being globally installed.

Modify the avr-lit executable so that it injects PATH=$(dirname ARGV[0]);$PATH.