refactor: name site tests by subject

The site test modules were cut by review pass (test_site_r2_css to test_site_r5_wiki): the passes of one subject become one module, and the modules that stood alone take the name of what they hold. The installer test classes named after a review take the name of what they test. One test left --manifest set for the tests after it, which the new order showed: it now resets the sources.

scripts/generate_site.py

@@ -3248,7 +3248,7 @@ def with_setup_table(page: str, coverages: dict, registry: dict | None = None) -
 
 # A platform's file lists are split by the share of a page they render to,
 # in thousandths: their elements against validate_site's MAX_PAGE_ELEMENTS,
-# their bytes against the 600,000 of test_site_r2_gate, whichever is
+# their bytes against the 600,000 of test_site_built_gate, whichever is
 # larger. Measured in Markdown characters, the lists split anew whenever
 # their markup changed: rows written as tables moved 38 more systems to
 # pages of their own.