Tests

Bump third_party/imgui/repo from `3c435c0` to `1d88609` #2300

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 2m 48s
Test Suites - Darwin (no-ble-asan-clang)	1h 41m 13s
Test Suites - Darwin (no-ble-tsan-clang)	1h 53m 13s
Test Suites - Linux (no-ble-tsan-clang)	1h 38m 54s
	6h 16m 8s

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