MultiTestDB: add an environment variable for overriding test-DB prefix

Until now, MultiTestDB would prefix the names of test databases it
creates with the local user name as determined by looking at the
environment variables: LOGNAME and USER, in the order as written here.
There are the following problems with this approach:
 - it leaks local user names to the database system and by extension to
   everyone who can access them,
 - in the event of said local user names being different from farm ones
   it makes it more difficult to identify to whom a specific database
   belongs.
To address these issues the present commit introduces a new environment
variable called ENSEMBL_TESTDB_PREFIX which when defined takes
precedence over LOGNAME. Now anyone running tests on their local machine
can set that variable to their farm user name.