Tests

Bump third_party/openthread/repo from `837c7fd` to `a8afffb` #905

Job	Run time
Test Suites - Linux (no-ble-tsan-clang)	1h 22m 46s
REPL Tests - Linux	28m 16s
Test Suites - Darwin (no-ble-asan-clang)	1h 24m 14s
Test Suites - Darwin (no-ble-tsan-clang)	1h 45m 14s
REPL Tests - Darwin	0s
	5h 0m 30s

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