platformio unity test cleanup

[malachib]

Remove symlinks under test/arduino/unity/lib/estdlib and instead point platformio.ini file directly at our own file system

Although they work in many scenarios, symlinks have two issues:

1. Not as well behaved on Windows systems
2. Goofs up global searches, doubling up responses

Hopefully solution for this one also applies to Arduino examples ostream and friends as well, though not part of the requirement

[malachib]

Other fun side effect: VSCode C++ extension appears to go into infinite and intense indexing loop

[malachib]

https://github.com/platformio/platformio-core/issues/5154