make test: create virtualenv under /test/

instead of using build-root, use /test/venv directory for virtualenv
similarly, don't pollute build-root with test-built binaries

Change-Id: I1e63c04037eaee718b27b34ef16c9eb0252afa53
Signed-off-by: Klement Sekera <ksekera@cisco.com>

test/test_vcl.py

@@ -33,7 +33,7 @@ class VCLTestCase(VppTestCase):
     """ VCL Test Class """
 
     def __init__(self, methodName):
-        var = "VPP_TEST_BUILD_DIR"
+        var = "VPP_BUILD_DIR"
         self.build_dir = os.getenv(var, None)
         if self.build_dir is None:
             raise Exception("Environment variable `%s' not set" % var)