EnumerateEnvironmentVariables test failed in CI

[stephentoub]

https://ci.dot.net/job/dotnet_corefx/job/master/job/windows_nt_debug_prtest/3862/consoleText

     System.Tests.GetEnvironmentVariable.EnumerateEnvironmentVariables(target: Process) [FAIL]
        Assert.Equal() Failure
        Expected: false
        Actual:   (null)
        Stack Trace:
           D:\j\workspace\windows_nt_de---4526f5ff\src\System.Runtime.Extensions\tests\System\Environment.GetEnvironmentVariable.cs(199,0): at System.Tests.GetEnvironmentVariable.EnumerateEnvironmentVariables(EnvironmentVariableTarget target)

cc: @JeremyKuhne

[JeremyKuhne]

Urgh, another test timing problem. There is no guarantee the value we iterate will still exist (or not be changed in some other way) by the time we look it up again. In this case I'm 99% sure that we're colliding with the other test classes.

I'll change this to set a specific variable and only compare that one.

On a related note it is unfortunate that Assert.Equal() doesn't allow adding additional context.

[karelz]

@JeremyKuhne I marked the bug with labels and milestone on your behalf.

[stephentoub]

https://ci.dot.net/job/dotnet_corefx/job/master/job/portablelinux_release_prtest/1204/consoleText

[jkotas]

https://ci.dot.net/job/dotnet_corefx/job/master/job/ubuntu14.04_release_prtest/3946/

[sepidehkh]

https://ci.dot.net/job/dotnet_corefx/job/master/job/ubuntu14.04_debug_prtest/3937/

[JeremyKuhne]

Fixed with dotnet/corefx#16546