Add resource intensiveness as a reason to not running tests by default

Hi,

With f47ed79cc8, the test suite doesn't run 'wal_consistency_checking'
as default because it is resource intensive; but regress docs doesn't
state resource intensiveness as a reason for not running tests by
default. So, I created a patch for updating the docs.

Any kind of feedback would be appreciated.

Regards,
Nazir Bilal Yavuz
Microsoft