tests: check that mysql_config_editor generates key with the 3 high bits
tests: check that mysql_config_editor generates key with the 3 high bits

Let's check (well, fuzz) that mysql_config_editor never sets the 3 high
bits of keys it generates.

The test is limited to 800ms. Let's see how many checks Travis-CI can do
in this timespan...

dolmen-go/mylogin