[WASM] Environment.UserName raises "not supported" exception

[safern]

Update: UserName now fails with System.IO.IOException : Not supported

This is causing System.Runtime.Extensions.Tests to crash.

Crashes with:

``` at 205708 (dotnet.js:1858:32) at _emscripten_asm_const_iii (dotnet.js:1863:27) at wasm_logger (:wasm-function[7791]:0x1f3d0d) at eglib_log_adapter (:wasm-function[6303]:0x1abc50) at monoeg_g_logstr (:wasm-function[7225]:0x1ced8d) at monoeg_g_logv_nofree (:wasm-function[7223]:0x1cecb4) at monoeg_g_logv (:wasm-function[7222]:0x1cec11) at monoeg_g_log (:wasm-function[7226]:0x1ceded) at getpwuid_r (:wasm-function[1131]:0x6e9e5) at SystemNative_GetPwUidR (:wasm-function[7726]:0x1effbc) ```

I guess this API should throw PNSE in WASM?

Will disable the tests against this issue in the meantime.

cc: @steveisok @lewing @marek-safar @akoeplinger

[akoeplinger]

Looks like on Xamarin iOS/Android we returned a dummy "somebody" string from Environment.UserName.

[akoeplinger]

#38256 fixed the crash but we're still reporting System.IO.IOException : Not supported from the property so we still need to fix that.

We should probably return the same dummy value that we return for Xamarin.iOS/Android.