syz-runtest: disable csource tests for HostFuzzer targets
syz-runtest: disable csource tests for HostFuzzer targets

HostFuzzer targets require that we cross-compile the csource tests and
then copy them to the target system. The code to copy files is
currently missing from syz-runtest; also, at least for Fuchsia (which
uses HostFuzzer mode), cross-compiling is non-trivial.

google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller