[libFuzzer] Increase timeouts on fork tests and skip one on aarch64

The tests fork.text, fork.sigusr.test and fork-ubsan.test intermittently
fail on the aarch64 buildbots. Input gathered from the fork.sigusr.test
implies that when the builder is under load the timeout value is not
sufficient. The fork-ubsan.test doesn't have a timeout and I think is not
always finding the error after 10000 runs so I've marked it as unsupported
for now.

Differential Revision: https://reviews.llvm.org/D61449

llvm-svn: 360126