Add test for pwrite with append flag

[yagehu]

WASI specifies fd_pwrite to be similar to POSIX pwritev which specifies that it should ignore the append flag and write from the supplied offset. Currently, only Wasmtime implements this behavior.

NB: There is a documented Linux bug that pwrite does not respect the offset and instead appends. Wasmtime fixes this by tracking the append flag status and hiding it from the OS.

[yagehu]

I've filed an issue with the WasmEdge team. They have expressed desire to bring WasmEdge behavior in line with Wasmtime and add this test case.

[loganek]

Thanks, the test looks reasonable. Could you please fix formatting problems reported by the build?

[yagehu]

@loganek Thanks! Fixed format.