Tests

Bump third_party/openthread/repo from `d4155e4` to `b73b124` #618

Job	Run time
REPL Tests - Linux	35m 31s
Test Suites - Darwin (no-ble-asan-clang)	48m 46s
Test Suites - Linux (no-ble-tsan-clang)	1h 21m 52s
REPL Tests - Darwin	0s
	2h 46m 9s

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