Makefile: Introduce makefile to ease testing

Refs. swh/infra/sysadm-environment#4794

Makefile

+IMAGE="helmunittest/helm-unittest:3.11.1-0.3.0"
+# requires --user in the docker call to allow writing in the user's home
+UID=1000
+# This allows to introspect the swh/tests/__snapshot__/<generated-chart>.yaml
+# output of the chart execution in the test context. It's in a dedicated
+# target as this generates temporary files. This is to be used exceptionally
+# to ease troubleshooting
+ACTIVATE_SNAPSHOT=--update-snapshot
+
+test:
+	docker run -ti --user $(UID) --rm -v $(PWD):/apps \
+	  $(IMAGE) swh
+
+test-with-snapshot:
+	docker run -ti --user $(UID) --rm -v $(PWD):/apps \
+	  $(IMAGE) $(ACTIVATE_SNAPSHOT) swh