make archiver tests run for the python source as well as for the pyinstaller binary, fixes #215
make archiver tests run for the python source as well as for the pyinstaller binary, fixes #215

if the borg.exe binary is not available in PATH, binary tests are skipped.

source tests are run without forking (for better speed, esp. on travis).
binary tests need forking the binary, of course.

for source tests, some tests check for an exception to happen.
for a forked binary, we of course can only check the exit code, which is non-zero in that case.

raxenak/borg,raxenak/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgimar/borg,raxenak/borg,raxenak/borg,edgewood/borg,mhubig/borg,mhubig/borg,edgimar/borg,edgewood/borg,edgewood/borg,edgimar/borg,edgewood/borg,edgimar/borg,mhubig/borg,RonnyPfannschmidt/borg