bindings/python: Fix call to create a symlink

[chu11]

In issue #1748, I made a blunt cut & paste error. There were no additional tests either, which is why it wasn't caught. Added a fix, and added some coverage tests.

[codecov-io]

Codecov Report

Merging #1934 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1934      +/-   ##
==========================================
+ Coverage   80.04%   80.08%   +0.03%     
==========================================
  Files         195      195              
  Lines       35066    35066              
==========================================
+ Hits        28070    28084      +14     
+ Misses       6996     6982      -14

Impacted Files	Coverage Δ
src/broker/modservice.c	78.84% <0%> (-0.97%)	⬇️
src/cmd/flux-module.c	83.72% <0%> (-0.24%)	⬇️
src/modules/kvs/kvs.c	66.78% <0%> (-0.15%)	⬇️
src/cmd/flux-event.c	77.97% <0%> (ø)	⬆️
src/modules/connector-local/local.c	74.81% <0%> (+1.18%)	⬆️
src/common/libflux/response.c	82.71% <0%> (+1.23%)	⬆️
src/modules/barrier/barrier.c	78.62% <0%> (+2.06%)	⬆️
src/common/libutil/veb.c	98.85% <0%> (+2.85%)	⬆️

[garlick]

Good catch!