`test_unknown_boards_share_the_default_pair` required every unrecognised
FQBN to collapse onto the default (esp32, esp32) key. 82f25a03 stopped
that on purpose: `_idf_target` defaults ANY unknown board to 'esp32', so
an AVR / RP2040 / STM32 compile inherited the ESP32 identity and queued
behind unrelated ESP-IDF builds it shares no build dir with. The test
was never updated, so master has been red since.
Replaced with the contract the code actually implements, split so a
future change cannot quietly take the useful half away:
- non-ESP32 FQBNs get distinct identities and distinct locks (an AVR
build must not wait on an ESP-IDF one);
- a non-ESP32 board still serialises with ITSELF — dropping the shared
key must not drop the per-board lock;
- an unknown `esp32:` FQBN still shares the default pair, because that
one really does land in the shared build dir, and there
over-serialising is the safe direction.
570b7e174c