golangci.yml: add rule to not run errcheck on test

Test files have some calls that don't necessarily get checked since they
have a very low probability to fail, and in a testing environment it's
acceptable to not have bulletproof code.

Therefore, we add a rule to not run that check on any file that ends
with `_test.go` (Go test convention), so the linter isn't as noisy as it
used to be.

.golangci.yml

@@ -20,6 +20,9 @@ issues:
     - linters:
         - lll
       source: "^//go:generate "
+    - linters:
+        - errcheck
+      path: ".*_test.go"
 
   # Maximum issues count per one linter. Set to 0 to disable. Default is 50.
   max-issues-per-linter: 0