Update unix-test-instructions.md

Updated run coreclr tests instructions for Linux. Included the new semicolon separated paths for the coreFxBinDir.

Author: LLITCHEV

Documentation/building/unix-test-instructions.md

@@ -36,7 +36,7 @@ Run tests:
 >     --testNativeBinDir=~/coreclr/bin/obj/Linux.x64.Debug/tests
 >     --coreClrBinDir=~/coreclr/bin/Product/Linux.x64.Debug
 >     --mscorlibDir=/media/coreclr/bin/Product/Linux.x64.Debug
->     --coreFxBinDir=~/corefx/bin/Linux.AnyCPU.Debug
+>     --coreFxBinDir=~/corefx/bin/Linux.AnyCPU.Release;~/corefx/bin/Unix.AnyCPU.Release;~/corefx/bin/AnyOS.AnyCPU.Release
 >     --coreFxNativeBinDir=~/corefx/bin/Linux.x64.Debug
 > ```