Teach build bot how to run LLDB WebKit tests.
I thought these tests were included in webkitpy tests? That would mean they are already run in buildbot, according to test outputs like <https://build.webkit.org/builders/Apple%20High%20Sierra%20Release%20WK2%20%28Tests%29/builds/6544/steps/webkitpy-test/logs/stdio>, we're running all of the lldb_webkit_unittest* tests.
Oh, I didn't see <https://bugs.webkit.org/show_bug.cgi?id=187916>.
<rdar://problem/50783835>