scripts/resulttool: add mention about new detected tests

Some regression reports show a lot of "PASSED->None" transitions. When such
big lot of identical transitions are observed, it could be that tests are
now failing, but it could also be that some tests has been renamed.

To detect such case, add a log in regression report to report the number of
new tests (i.e: tests that are present in target results but not in base
result). This new log also allows to know about newly added tests bases

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>