test: allow skipping individual WPT subtests #4878

Job	Run time
Build slim tarball	3m 2s
aarch64-linux: with shared libraries	19m 5s
aarch64-darwin: with shared libraries	17m 16s
x86_64-linux: with shared libraries	22m 2s
x86_64-darwin: with shared libraries	45m 19s
	1h 46m 44s

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