util/regress: make default action a more thorough regression

util/regress: make default action a more thorough regression

Changed the --variants option to --test-variants and added a new
--compile-variants option for variants that are only compiled
(not tested).  The former still defaults to 'opt' and the latter
defaults to 'debug,fast'.

Also changed the behavior when no tests are specified from just
compiling to running the 'quick' tests.

As a result, a plain 'util/regress' invocation will now compile
(but not test) the debug and fast builds, and compile and run the
quick regressions on the opt build.  This should be the default
set of tests that are run before committing.  Since the nightly
regressions use this same script, this will also be the new
nightly regression behavior.

Test-only regressions can still be done by setting --compile=''.
Compile-only regressions can be done by setting --test=''.