[Transpose & generate_tests.py]: Added join env Function & Changed Template

[BethanyG]

As a result of the discussion here in the forum and the previously logged issue here in GitHub, decided to alter the JinJa2 template for this exercise, so that the test case input was "pre joined" prior to verifying it in the test case.

This is to avoid any ambiguity around whether the function input is intended to be a list or a str (the intent from problem specs is that the inputs are strs).

[x] Changed generate_tests.py to add the join_test_inputs() function & make it available in the template environment.
[x] Changed the JinJa2 template to use the pre-join functionality, and to preserve the \n characters.
[x] Changed the stub to reflect renaming lines to text for the input parameter.
[x] Regenerated a new test file & tested example.py against it.

[github-actions]

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