Makefile: Alter VPP_PYTHON_PREFIX for preloading deps

Allow setting of VPP_PYTHON_PREFIX to alternate location
so the python prereqs can be installed into base image
Also added test-dep trigger to isolate dependency install
from actual test run

Change-Id: Ia80f5dbf71bc24eb46cd6586bcadd474ef822704
Signed-off-by: Ed Kern <ejk@cisco.com>
diff --git a/test/Makefile b/test/Makefile
index 7984e31..cbeec79 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -128,6 +128,8 @@
 ext:
 	make -C ext
 
+test-dep: verify-python-path $(PAPI_INSTALL_DONE)
+
 test: verify-python-path $(PAPI_INSTALL_DONE) ext sanity reset
 	$(call retest-func)