serialize: Ignore two flaky json tests on windows

See #14064 for some rationale, but the basic idea is that I suspect that there
is an LLVM codegen bug somewhere, and I'm not entirely sure why it's happening
intermittently rather than deterministically...

cc #14064
auto merge of #14065 : alexcrichton/rust/ignore-flaky-windows-test, r=brson

See #14064 for some rationale, but the basic idea is that I suspect that there
is an LLVM codegen bug somewhere, and I'm not entirely sure why it's happening
intermittently rather than deterministically...

cc #14064