testing/yosys: Build and use internal abc
Fixes this error:
2.41.13.5. Executing ABC9. Running ABC command: "abc" -s -f /abc.script 2>&1 ABC: sh: abc: not found ERROR: ABC: execution of command ""abc" -s -f /tmp/yosys-abc-aDiLnD/abc.script 2>&1" failed: return code 127.
... when "synth_ice40 -abc9" is used.
This MR is an alternative to !58812 (merged) (not necessarily better or worse, I'll leave that for others to decide).