Even since 4632629 (Detect python executable path for tests too.) the
test suite failed to pass on systems where python3 is the default
executable for "python". This happened because test/ctags_stub.py would
still use the default of /usr/bin/python3, since environment variable
updates were only present during the config/make stages.
This changes test/ctags_stub.py to have an autogenerated shebang (#!)
line using automake. It resolves build issues by forcing use of the
python version chosen during configure regardless of the python
environment variable chosen at runtime.