Abseil's own tests now are disabled if either BUILD_TESTING
or a new option called ABSL_BUILD_TESTING is false.
Additionally, Abseil's CMakeLists.txt no longer re-declares
the BUILD_TESTING option with a value of false.

Abseil had been using just the BUILD_TESTING option, since
the fix for #901.  Because setting BUILD_TESTING false still
works to disable Abseil's tests, this change preserves the
behavior asked for in that issue.

Previous to that, Abseil had a project specific flag for
this, as is the typical idiom used in other projects.

The issue with BUILD_TESTING is that it is an all-or-nothing
policy.  When Abseil is incorporated as a subproject, the
encompasing project has no convenient way to enable its own
tests while disabling Abseil's.

Fixes #1056