MDL-67673 phpunit: Disable phpunit results cache by default

It can be enabled if desired from CLI using the --cache-result
option (or modifying the phpunit.xml cacheResult to true).

That will create the .phpunit.result.cache files that,
later, can be used to easily repeat failed (defects) tests or
run the slow ones first and more. See the --order-by option
to know more about all the available criteria. For example:

./vendor/bin/phpunit --order-by=defects --stop-on-defect

(will run all the failed ones first, stopping on problem, useful
for TDD and other scenarios, not so much for complete runs or CI).

Also, added to .gitignore so they won't be committed ever.

.gitignore

@@ -33,6 +33,7 @@ CVS
 /.project
 /.buildpath
 /.cache
+.phpunit.result.cache
 phpunit.xml
 # Composer support. Do not ignore composer.json, or composer.lock. These should be shipped by us.
 composer.phar

phpunit.xml.dist

@@ -9,6 +9,7 @@
         processIsolation="false"
         backupGlobals="false"
         backupStaticAttributes="false"
+        cacheResult="false"
         stopOnError="false"
         stopOnFailure="false"
         stopOnIncomplete="false"