Add test for gainmap #118

Job	Run time
windows (5.1, luajit, luajit)	2m 56s
windows (5.4, lua, lua)	3m 1s
unix (5.3, ubuntu-latest)	51s
unix (5.4, macos-latest)	1m 53s
unix (5.4, ubuntu-latest)	1m 2s
unix (5.2, ubuntu-latest)	52s
unix (luajit-openresty, macos-latest)	2m 24s
unix (5.1, macos-latest)	2m 7s
unix (luajit-openresty, ubuntu-latest)	51s
unix (5.1, ubuntu-latest)	49s
unix (5.3, macos-latest)	2m 7s
unix (5.2, macos-latest)	2m 30s
windows (5.3, lua53, lua5.3)	3m 4s
	24m 27s

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