Add information on using run-test.sh outside CI

Author: ellismg

Documentation/building/cross-platform-testing.md

@@ -0,0 +1,58 @@
+# Running XUnit tests cross platform
+
+Unlike Windows, where we run tests as part of the build, we have a seperate
+explicit testing step on Linux and OSX.  Over time, this special step will go
+away in favor of a similar "run tests during the build" model.
+
+`run-test.sh` is the shell script used by Jenkins in order to run all the XUnit
+tests cross platform. It combines the cross platform CoreCLR and CoreFX builds
+together into a test layout and then runs each test project from CoreFX.
+
+In order to run tests, you need to build a bunch of different projects.  The
+instructions assume you are building for Linux, but are easily modifiable for OSX.
+
+1. Release or debug CoreCLR.  In Jenkins we use a release CoreCLR build instead
+   of debug CoreCLR since it is much faster at actually running XUnit, but debug
+   will work if you have the time.
+
+   From the root of your CoreCLR enlistment on Linux, run `./build.sh Release` in
+   order to build.
+2. A coresponding version of mscorlib.dll, built on Windows but targeting
+   Linux.  This can be produced by running `build.cmd linuxmscorlib Release` from
+   a CoreCLR enlistment on Windows.  Remember that the runtime and mscorlib are
+   tightly coupled with respect to object sizes and layout so you need to ensure
+   you have either a release coreclr and release mscorlib or debug coreclr and
+   debug mscorlib.
+3. A Linux build of CoreFX.  On Windows, run `build.cmd /p:OSGroup=Linux`.  It
+   is okay to build a Debug version of CoreFX and run it on top of a release
+   CoreCLR (which is exactly what we do in Jenkins).
+4. A Linux build of the native CoreFX components.  On Linux, run ./build.sh from
+   src/Native in your CoreFX enlistment.
+
+After building all the projects, we need to copy the files we built on Windows
+over to our Linux machine.  The easiest way to do this is to mount a windows
+share on linux.  For example, I do:
+
+```
+# sudo mount.cifs "//MATELL3/D\$" ~/mnt/matell3/d -o user=matell
+```
+
+The copy the CoreFX binaries over to your local machine (I have my enlistements
+on Linux under ~/git).
+
+```
+# rsync -v -r --exclude 'obj' --exclude 'packages' ~/mnt/matell3/d/git/corefx/bin/ ~/git/corefx/bin/
+# rsync -v -r  ~/mnt/matell3/d/git/coreclr/bin/Product/ ~/git/coreclr/bin/Product/
+```
+
+Then, run the tests.  run-test.sh defaults to wanting to use Windows tests (for
+historical reasons), so we need to pass an explict path to the tests, as well as
+a path to the location of CoreCLR and mscorlib.dll.
+
+```
+# ./run-test.sh --coreclr-bins ~/git/coreclr/bin/Product/Linux.x64.Release \
+--mscorlib-bins ~/git/coreclr/bin/Product/Linux.x64.Release \
+--corefx-tests ~/git/corefx/bin/tests/Linux.AnyCPU.Debug
+```
+
+run-test.sh should now invoke all the managed tests.