config init now leaves out the test sources unless given --all

A section tagged init:"hidden" isn't written to the file config init
generates; fts_mock and sleeper are. config init --all writes them too.
They still load, and the config commands read and set them as before.

internal/config/README.md

@@ -267,6 +267,10 @@ per key:
   of the configuration.
 - `default:"value"` is the field's default, parsed as if it were a string
   passed to `SetValue` (e.g. `"10m"`, `"true"`).
+- `init:"hidden"` on a nested section's field leaves the section out of the
+  file `config init` writes, unless `--all` is given (`InitDefaults`). The
+  test sources, `fts_mock` and `sleeper`, have it. The section is otherwise
+  as any other: loaded, and read and set by the `config` commands.
 - A tagged field whose type is a struct is a nested section, addressed as
   `section.key`. An embedded struct has its fields promoted, as Go does;
   every source embeds `JSCSourcesSource` for `enabled` and `interval`.
@@ -324,7 +328,8 @@ To add a new section:
    `JSCRateLimitRule`, has no constant and passes `""` to `flattenOf`.
 2. Add the struct as a tagged field of its parent: `JobSearchConfig` in
    `config.go`, or `JSCSources` in `conf_sources.go` for a source,
-   etc. The field's `cfg` tag must match the `JSCK*` constant.
+   etc. The field's `cfg` tag must match the `JSCK*` constant. Add
+   `init:"hidden"` if it's only for testing.
 
 Nothing in `section.go` needs changing.