Reuses async_run parallel group for health-checker tasks. #548

Job	Run time
B2 msvc-14.2	8m 33s
B2 gcc-11	5m 26s
CMake msvc-14.2 Debug C++17	8m 35s
B2 clang-14	5m 24s
B2 msvc-14.3	7m 14s
CMake gcc-11 17 Debug	7m 47s
CMake msvc-14.2 Release C++17	7m 51s
CMake gcc-11 20 Release	10m 54s
CMake msvc-14.3 Debug C++20	8m 13s
CMake clang-11 17 Debug	6m 26s
CMake msvc-14.3 Release C++20	8m 16s
CMake clang-11 20 Debug	6m 31s
CMake clang-13 17 Release	9m 10s
CMake clang-13 20 Release	9m 40s
CMake clang-14 17 Debug -stdlib=libc++	7m 26s
CMake clang-14 20 Release -stdlib=libc++	11m 3s
	2h 8m 29s

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