testsuite: make 'stats' tests more flexible about the location of timing files

To accomplish this, this patch introduces a stats_files_dir configuration field in the testsuite driver where all haddock timing files 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.

This also lets us get rid of the extra_files directives in the all.T entries for haddock.base, haddock.Cabal and haddock.compiler.