Move Pester to submodule to Host/Modules

This way it can also be deployed automatically, removing the last manual
copy steps from our build scripts.

Travis and AppVeyor configurations updated for new submodule location.

.travis.yml

@@ -5,7 +5,7 @@ git:
   submodules: false
 before_install:
   - git config --global url.git@github.com:.insteadOf https://github.com/
-  - git submodule update --init --recursive -- src/libpsl-native src/monad src/windows-build test/Pester
+  - git submodule update --init --recursive -- src/libpsl-native src/monad src/windows-build src/Microsoft.PowerShell.Linux.Host/Modules/Pester
   - sudo sh -c 'echo "deb [arch=amd64] http://apt-mo.trafficmanager.net/repos/dotnet/ trusty main" > /etc/apt/sources.list.d/dotnetdev.list'
   - sudo apt-key adv --keyserver apt-mo.trafficmanager.net --recv-keys 417A0893
   - sudo apt-get -qq update