pickler: Fix failing tests

Some logic may have crossed which caused the make_refs branch to fail
its tests.  The reason was that _mkrefs() changed to return either
objects or reference objects, but one code path was still returning
`bool`, which was its original behavior.

The unpickable condition has now been folded into the make_refs
condition which decreases complexity and fixes the tests.

Signed-off-by: David Aguilar <davvid@gmail.com>
runtests.py: Fix sys.path handling

Prepend into sys.path so that the source repo is preferred over other
paths.  This avoids failing tests when jsonpickle is already installed
in the default site-packages directory.

Signed-off-by: David Aguilar <davvid@gmail.com>
tests: Remove a leftover print statement and shorten lines per PEP-8

Signed-off-by: David Aguilar <davvid@gmail.com>