Fix building and testing with the JS backend

Note that most tests have been disabled because they don't pass due
to missing C function support (e.g. pipe). But it's a good basis to
start adding tests for supported things (e.g. touchFd, cf #285).

This requires a recent version of GHC that provides
getMonotonicTimeNSec. See
ghc/ghc#23687 and
ghc/ghc!10900.