Fix unit tests. test_layer: reverted material for add_potting_region …

[ansjmoody]

…from COPPER to epoxyencapsulant. test_stackup: changed board_dimensino in validation of get_stackup_props()

Checklist:

• Run unit tests and make sure they all pass
• Check and fix style errors
  - pre-commit command line check
  - Problems tab in PyCharm
• [na] Bench test new/modified APIs by using and modifying the code in the example for the API method
• [na] Add new methods to rst file for updated API script(\doc\source\api)
• [na] Generate documentation
• [na] Verify the HTML. It gets generated at: \doc\build\html.
  - Open index.html
  - Click on "API Reference" at the top.
  - Verify HTML for API changes.
• Check that test code coverage is at least 80% when Sherlock is running and can be connected to using gRPC

[codecov]

Codecov Report

Merging #179 (5d24d1c) into main (9e59652) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #179   +/-   ##
=======================================
  Coverage   56.10%   56.10%           
=======================================
  Files          20       20           
  Lines        3212     3212           
=======================================
  Hits         1802     1802           
  Misses       1410     1410