Assert error on ambiguous fstflags

[yagehu]

This commit adds a Rust test case that asserts an ambiguous fstflags input to fd_filestat_set_times should result in inval errno. An fstflags argument is ambiguous if:

1. both atim and atim_now are set, or
2. both mtim and mtim_now are set.

This assertion is consistent with Wasmtime and WasmEdge's current behavior.