ci: exclude test helpers and generated code from the coverage report

The coverage profile counted the tests/ package and the *_gen.go files, none of
which are code under test: tests/ is the mock and helper package, and generated
code is never hand-tested. Together they added 2926 uncounted statements at 0%,
pulling the reported number down by almost 6 points (70.13% -> 75.98% on the
current master profile).

octocov's coverage.exclude takes doublestar globs matched against git-root-relative
paths. All 26 mock_*.go files live under tests/, so the single 'tests/**' pattern
covers them.

.octocov.yml

@@ -7,6 +7,10 @@ coverage:
   # switches to line counting when it merges several itself.
   paths:
     - coverage.out
+  # Not code under test: tests/ holds the mocks and helpers, *_gen.go is generated.
+  exclude:
+    - 'tests/**'
+    - '**/*_gen.go'
 codeToTestRatio:
   # Needs the pull request's own source, which the comment workflow must not
   # check out: it holds a write token.