CMake: improve handling of `gecode-test` #1

Job	Run time
build-autoconf (macos-latest)	2m 55s
build-cmake (macos-latest)	5m 0s
build-autoconf (ubuntu-latest)	7m 56s
build-cmake (ubuntu-latest)	9m 59s
build-cmake (windows-latest)	10m 50s
	36m 40s

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