ci: Test Emacs 26 and 27 #280

Job	Run time
test (ubuntu-latest, 26.3)	35s
test (ubuntu-latest, 27.2)	1m 4s
test (ubuntu-latest, 28.2)	1m 26s
test (ubuntu-latest, 29.4)	56s
test (macos-latest, 28.2)	2m 37s
test (macos-latest, 29.4)	2m 23s
test (windows-latest, 26.3)	2m 49s
test (windows-latest, 27.2)	1m 52s
test (windows-latest, 28.2)	2m 16s
test (windows-latest, 29.4)	2m 52s
test (ubuntu-latest, snapshot, true)	37s
test (macos-latest, snapshot, true)	2m 4s
test (windows-latest, snapshot, true)	3m 26s
	24m 57s

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