bpo-30283: Add NEWS entry for backported regrtest (#2398)

Misc/NEWS.d/next/Tests/2017-06-26-11-24-14.bpo-30283.qCQmlm.rst

+regrtest: Enhance regrtest and backport features from the master branch.
+
+Add options: --coverage, --testdir, --list-tests (list test files, don't run
+them), --list-cases (list test identifiers, don't run them, :issue:`30523`),
+--matchfile (load a list of test filters from a text file, :issue:`30540`),
+--slowest (alias to --slow).
+
+Enhance output: add timestamp, test result, currently running tests, "Tests
+result: xxx" summary with total duration, etc.
+
+Fix reference leak hunting in regrtest, --huntrleaks: regrtest now warms up
+caches, create explicitly all internal singletons which are created on demand
+to prevent false positives when checking for reference leaks.
+(:issue:`30675`).