add standard "check" target

The wider build world uses `make check` to run all unittests.
Add that target to our build too.

Change-Id: I55f867e75ea8c01fd6a6e121d3f393403a8d9d34
Reviewed-on: https://weave-review.googlesource.com/3150
Reviewed-by: Alex Vakulenko <avakulenko@google.com>

tests.mk

diff --git a/tests.mk b/tests.mk
index 35557d7..a02100b 100644
--- a/tests.mk
+++ b/tests.mk
@@ -64,6 +64,7 @@
 	$(TEST_ENV) $< $(TEST_FLAGS)
 
 testall : test export-test
+check : testall
 
 ###
 # coverage
@@ -81,5 +82,4 @@
 
 coverage: run_coverage
 
-.PHONY : coverage run_coverage test export-test testall
-
+.PHONY : check coverage run_coverage test export-test testall