TESTING.md: document GO_VERSION

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

TESTING.md

@@ -87,3 +87,10 @@ To run the integration test suite:
 ```
 make test-integration
 ```
+
+You can change a version of golang used for building stuff that is being tested
+by setting `GO_VERSION` variable, for example:
+
+```
+make GO_VERSION=1.12.7 test
+```