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Update core-test submodule & Drop Emacs 26 support #332

Job	Run time
test (ubuntu-latest, 28.2, false)	43s
test (ubuntu-latest, 29.3, false)	1m 6s
test (macos-latest, 28.2, false)	1m 37s
test (macos-latest, 29.3, false)	2m 38s
test (windows-latest, 28.2, false)	1m 44s
test (windows-latest, 29.3, false)	3m 17s
test (ubuntu-latest, 26.3, false)	40s
test (windows-latest, 26.3, false)	3m 45s
test (ubuntu-latest, 27.2, false)	41s
test (windows-latest, 27.2, false)	1m 19s
test (ubuntu-latest, snapshot, true)	40s
test (macos-latest, snapshot, true)	1m 32s
test (windows-latest, snapshot, true)	4m 38s
	24m 20s

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