tests: pin pip version

New pip releases can break pip-tools. This commit pins the version of
pip used in the test virtualenv to prevent uncontrolled breakage.

This fixes the current issue in make test:
TypeError: __init__() got an unexpected keyword argument 'index_urls'

Change-Id: I3b7ecb31e651401ada0d357e2bf093c91c934565
Type: feature
Signed-off-by: Aloys Augustin <aloaugus@cisco.com>
diff --git a/test/Makefile b/test/Makefile
index e2823ee..8a6d69c 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -80,6 +80,7 @@
 endif
 
 PYTHON_VERSION=$(shell $(PYTHON_INTERP) -c 'import sys; print(sys.version_info.major)')
+PIP_VERSION=19.1.1
 PIP_TOOLS_VERSION=3.8.0   # Keep in sync with requirements.txt
 PYTHON_DEPENDS=requirements-$(PYTHON_VERSION).txt
 SCAPY_SOURCE=$(shell find $(VENV_PATH)/lib/$(PYTHON_INTERP) -name site-packages)
@@ -96,6 +97,9 @@
 	@rm -rf $(VENV_PATH)
 	@mkdir -p $(TEST_RUN_DIR)
 	@virtualenv $(VENV_PATH) -p $(PYTHON_INTERP)
+	# pip version pinning
+	@bash -c "source $(VENV_PATH)/bin/activate && \
+		  $(PYTHON_INTERP) -m pip install pip===$(PIP_VERSION)"
 	@bash -c "source $(VENV_PATH)/bin/activate && \
 		  $(PYTHON_INTERP) -m pip install pip-tools===$(PIP_TOOLS_VERSION)"
 	@touch $@