docs: Release notes for ptest updates

Signed-off-by: David Brown <david.brown@linaro.org>

docs/release-notes.d/ptest.md

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+- Update ptest to support test selection. Ptest can now be invoked with `list`
+  to show the available tests and `run` to run them. The `-t` argument will
+  select specific tests to run.
+- Allow sim tests to skip slow tests.  By setting `MCUBOOT_SKIP_SLOW_TESTS` in
+  the environment, the sim will skip two tests that are very slow.  In one
+  instance this reduces the test time from 2 hours to about 5 minutes.  These
+  slow tests are useful, in that they test bad powerdown recovery, but are
+  inconvenient when testing other areas.