scripts/runTests.fsx: acceptance test for warning #1518

Job	Run time
macOS--dotnet6-and-mono	6m 15s
linux-newLTS-github--dotnet-and-msbuild	5m 41s
linux-newLTS-vanilla--stockmono	4m 31s
linux-newLTS-github--dotnet-and-xbuild	5m 20s
linux-oldLTS-github--dotnet-and-msbuild	5m 36s
linux-oldLTS-github--dotnet-and-xbuild	5m 32s
linux-oldLTS-vanilla--newmono	5m 6s
linux-oldLTS-vanilla--stockmono	3m 49s
macOS--mono	4m 18s
linux-newLTS-vanilla--dotnet6	5m 28s
linux-newLTS-vanilla--newmono	4m 39s
windows--dotnet6	8m 39s
windows--legacyFramework	17m 17s
sanity-check	0s
publish	0s
	1h 22m 11s

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