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:

• test-frontend-full
• test-frontend-full-ui