Move core::run tests of process killing into standalone run-pass tests

These cause valgrind errors in subprocesses. I don't *think* these errors
lead to actual test failures but they are very confusing.
auto merge of #6228 : brson/rust/run-destroy, r=brson

These cause valgrind errors in subprocesses. I don't *think* these errors
lead to actual test failures but they are very confusing.