They are a particular type of perf tests. This patch introduces a
'stats_files_dir' configuration field in the testsuite driver where all
haddock timing files (and possibly others in the future) are assumed to live.
We also change both the Make and Hadrian build systems to pass respectively
$(TOP)/testsuite/tests/perf/haddock/ and
<build root>/stage1/haddock-timing-files/ as the value of that new
configuration field, and to generate the timing files in those directories
in the first place while generating documentation with haddock.

This new test type can be seen as one dedicated to examining stats files that
are generated while building a GHC distribution. This also lets us get rid of
the 'extra_files' directives in the all.T entries for haddock.base,
haddock.Cabal and haddock.compiler.