Run some of Haddock's tests in the testsuite

The 4 main testsuites in Haddock don't have many dependencies, but are regularly broken in small ways by changes to the GHC AST or the GHC API.

This adds four tests to the testsuite that build the four haddock suites (html-test, latex-test, hoogle-test, and hypsrc-test) and run to check that nothing fails in them.

Fixes #16206 (closed)