Add test dependency installation on Gentoo

Here I add test deps for Gentoo, the main ones to call out are as follows.

* pyyaml: not installed by default and needed for the package element
* parted: not installed by default and needed for the VM element
* multipath-tools: not installed by default and provides kpartx

Change-Id: I07ce871fb3e684bfd9d06268d5c5bd118314f321

tests/install_test_deps.sh

@@ -13,4 +13,9 @@ sudo apt-get install -y \
     sudo yum -y install \
         debootstrap \
         kpartx \
-        qemu-img
+        qemu-img || \
+    sudo emerge \
+        app-emulation/qemu \
+        dev-python/pyyaml \
+        sys-block/parted \
+        sys-fs/multipath-tools