[dotest] --skip-category should append and not override.
[dotest] --skip-category should append and not override.

<rdar://problem/38566150>

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@329358 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb