scripts: make_tb_path: Error for unset ADI_*_DIR

[gastmaier]

PR Description

Add informative error for mandatory environment variable.
I would rather "hardcode" the depth on the entry Makefile (the one that we can obtain the current shell path),
and infer these variables for most users.

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• New test (change that adds new test program and/or testbench)
• Breaking change (has dependencies in other repositories/testbenches)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have ran all testbenches affected by this PR
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Errors on compilation/elaboration/simulation
• I have set the verbosity level to none for the test program

[IstvanZsSzekely]

Really good suggestion!
Tested with Environment variables set and unset, both work as intended.