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.

modules/Bio/EnsEMBL/Test/MultiTestDB.pm

@@ -781,7 +781,11 @@ sub create_db_name {
   # Create a unique name using host and date / time info
   my $db_name = sprintf(
       '%s_test_db_%s_%s_%s_%s',
-      ( exists $ENV{'LOGNAME'} ? $ENV{'LOGNAME'} : $ENV{'USER'} ),
+      (
+        exists $ENV{'ENSEMBL_TESTDB_PREFIX'} ? $ENV{'ENSEMBL_TESTDB_PREFIX'}
+          : exists $ENV{'LOGNAME'}           ? $ENV{'LOGNAME'}
+          : $ENV{'USER'}
+      ),
       $species, $dbtype, $date, $time
   );
   if (my $path = $self->_db_path($self->dbi_connection)) {