Create global "execution date" parameter for @DisabledUntil

[nipafx]

The @DisabledUntil documentation contains this warning:

Applying @DisabledUntil can make the test suite non-reproducible.
If a failing test is disabled during a build that then passes, rerunning that build after the "until" date would fail.
A report entry is issued for every test that is disabled until a certain date.

This could be fixed by introducing a configuration parameter that @DisabledUntil interprets as "today", so tests that builds can be reproduced.

If we implement this, a no-longer-disabled test that fails should probably emit a message pointing this option behavior out.

Relates to #477.