workflows: run tests for master

Sometimes merged code != PR code and we want to have some proper statistics
for covertage as well (it doesn't have proper data for master now because
we're not collecting it).

Signed-off-by: Roman Khimov <roman@nspcc.ru>

.github/workflows/tests.yml

@@ -1,6 +1,11 @@
 name: neofs-sdk-go tests
 
 on:
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - '*.md'
   pull_request:
     branches:
       - master