tests: cover the bugs page access control and config

Exercise the C plumbing for the bugs page.  The tests need neither Lua
nor git-bug, since everything they check happens before the filter would
be invoked:

- the bugs tab is absent by default and present with enable-bugs=1
- enable-bugs is honoured both globally and per-repo
- the page returns 404 when no bugs-filter is configured
- the page returns 403 when it is not enabled
- a bug id in the path does not leak into the other nav tab URLs
- cache-bugs-ttl parses without complaint

Assisted-by: LLM [codegen, tests]
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>