tests: add vpp test dirs to PYTHONPATH

tests: add vpp test dirs to PYTHONPATH

- Required to relocate test infra modules
  to be co-located in feature test directory.
- Remove/regenerate build-root/build-test/src/
  soft links when running make test to allow
  re-running 'make test' after moving source files.
- Suppress annoying 'ls' error message when gathering
  test source files caused by 'c' test application
  directory being named 'test'.
- Use PYTHONPYCACHEPREFIX to generate all
  compiled source under build-root/build-test.
  This only works for python 3.8 or greater, but
  existing 'make test-wipe' target cleans up
  in-situ __pycache__ directories for older
  python versions.

Type: fix

Change-Id: If3c3ff0c58fb26a17395babc34783e06b1dd84e7
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>