build: set default target to test

A Makefile's default target is either the one defined by "all", or in
the absence of an "all" target, the first target found in the file.

This means the current default target is to clean the venv from disk.
This patch makes calling make non-destructive by redefining the default
target to run the tests.

We may want to change this in future to something more appropriate.

Fixes: GREG-13

Makefile

@@ -9,6 +9,8 @@ pip = python -m $(PIP)
 poetry = python -m $(POETRY)
 venv = . $(VENV)/bin/activate &&
 
+all: test
+
 .PHONY: clean
 clean:
 	$(RM) -r $(VENV)