PROTON-2413: [Python] set PATH in example test runner
PROTON-2413: [Python] set PATH in example test runner

Previously the test runner assumed it could find the examples on the
path. This meant that the environment running the examples would have to
set up the path correctly. This is unnecessary as they are all in the
same directory as the test runner itself.

astitcher/qpid-proton,apache/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,astitcher/qpid-proton,apache/qpid-proton,apache/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton