Do not include 'tests' module in default image

I assume this was required for `make check` (without image=) to work.
Let's have 'make check' use 'tests' image by default unless overwritten.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

Makefile

@@ -42,6 +42,8 @@ clean:
 	$(call quiet, rm -rf build/$(mode), CLEAN)
 	$(call only-if, $(mgmt), $(call quiet, cd mgmt && ./gradlew --daemon clean >> /dev/null, GRADLE CLEAN))
 
+check: export image ?= tests
+
 check: all
 	./scripts/test.py
 

images/default.py

@@ -2,4 +2,3 @@ from osv.modules.api import *
 
 _mgmt = require('mgmt')
 run = [ _mgmt.full ]
-require('tests')