Increase Makefile test timeout to 10 minutes

We have a number of Evergreen tests that timeout and fail; perhaps
giving them longer will let them succeed.

Change-Id: Id623338e57a057bd45bdfc8d9498dcd0644263a7

Author: xdg

Makefile

@@ -7,7 +7,7 @@ CORE_TEST_PKGS = $(shell ./etc/find_pkgs.sh ./core _test)
 PKGS = $(BSON_PKGS) $(MONGO_PKGS) $(CORE_PKGS)
 TEST_PKGS = $(BSON_TEST_PKGS) $(MONGO_TEST_PKGS) $(CORE_TEST_PKGS)
 
-TEST_TIMEOUT = 300
+TEST_TIMEOUT = 600
 
 .PHONY: default
 default: check-fmt vet build-examples lint errcheck test-cover test-race