Add `jetzig test` command which runs build step `jetzig:test`.
Add `jetzig.testing` namespace which provides test helpers and a test
app.
Add tests to view generator (i.e. include tests for generated routes).
Views are copied to zig-cache, meaning that any files in `src/` are not
available for `@import` by default. Copy all .zig files from `src/` into
zig-cache so that relative imports work as expected. This also removes
the need to specifically copy views into zig-cache as the full tree is
now present.
Also fix a bug in static route fallback - remove superfluous underscore
so static routes with non-matching params render default HTML/JSON
content.
