Update test GetProcesses_LongProcessName() for Alpine distro

[MarcoRossignoli]

Test System.Diagnostics.Tests.ProcessTests.GetProcesses_LongProcessName makes a copy and rename sleep command to test fix for https://github.com/dotnet/corefx/issues/34437

Unfortunately on Alpine distro sleep is implemented as a symlink to busybox

readlink $(command -v sleep) -> /bin/busybox

We should try to find a way to test also on this distro.
At the moment the best idea could be create a custom library like NativeLibrary test that will work on all platform.

/cc @danmosemsft @am11

[wfurt]

ping is "normal" executable in alpine-3.9-helix-09ca40b-20190620160617 image. I think we should update test to skip if ping is link instead of checking for particular distro type.
Note that it would be nice to use throw new SkipTestException() so we can track it as skip in test result database. (check HTTP how it is done)

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