The COMP_ settings of the last test of _count_args() were falling through to
the first test of _get_cword(), causing it to fail.
Unset COMP_ variables within the unit tests setup & teardown.
Fixed last test at `_count_args.exp' by solving conflicting settings of
COMP_CWORD and COMP_POINT.
Steps to reproduce the problem:
$ ./runUnit _count_args.exp _get_cword.exp
...
Running ./unit/_count_args.exp ...
Running ./unit/_get_cword.exp ...
ERROR Unexpected output from bash command "_get_cword should run without errors":
bash: $index: substring expression < 0
...
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