binary-search-tree : add test template

[tsamoglou]

No description provided.

[cmccandless]

A couple of things:

• You seem to be mixing template variables with Python variables. The rendered test code will not have access to variables like case or macros declared in the template (i.e. test_call)
• I noticed you were having difficulty with subcase groups nested with actual test cases (exercism/problem-specifications#1629). The proper way to deal with this in the template is to check for the cases field in the case object, and handle it accordingly.

With the above in mind, please see my refactored version here.

[cmccandless]

Feel free to duplicate my changes if you wish. If you would like to use my commit directly:

• If you checked "Allow edits from maintainers" when creating this pull request, I can push my commit directly to your branch.
• If not, I can guide you through pulling my commit from my fork into yours.

[tsamoglou]

@cmccandless I did that you say, the branch is ready for commit!

[cmccandless]

My apologies, my fix had an error that created an extra blank line. I determined I had the necessary permissions and took the liberty of pushing the fix myself.