cypress: Skip tests marked as slow by default
Some cypress tests can take a long time to execute while not significantly
improving overall code coverage (e.g. language highlighting tests).
So add a way to declare a tests suite to be slow to execute and skip it
by default (notably to speedup CI jobs). In order to execute it back,
the CYPRESS_SKIP_SLOW_TESTS environment variable must be set to a value
different from 1.
Two new make targets have been added in order to execute all tests even
the slow ones: