Switch from "use base" to a method which can run more than once

RT::Handle->FinalizeDatabaseType is called when the package is
require'd, thanks to the final lines of RT::Handle.  This means that it
is always called in RT::Test->bookstrap_db, which is always called on
'use RT::Test'.  Furthermore, "use base ..." will only have an effect
_once_.  As such, in t/web/install.t, the "SQLite" handle is actually
still an DBIx::SearchBuilder::Handle::Oracle despite DatabaseType being
SQLite.  This leads to the SystemDSN being
dbi:SQLite:sid=/path/to/rt4test_install_xxx;host=hostname;port=1521
which fares about as well as you might expect.

Switch to setting the parentage via an explicit require and @isa, which
can be called more than once.

lib/RT/Handle.pm

@@ -83,17 +83,17 @@ L<DBIx::SearchBuilder::Handle>, using the C<DatabaseType> configuration.
 =cut
 
 sub FinalizeDatabaseType {
-    eval {
-        use base "DBIx::SearchBuilder::Handle::". RT->Config->Get('DatabaseType');
-    };
-
     my $db_type = RT->Config->Get('DatabaseType');
-    if ($@) {
+    my $package = "DBIx::SearchBuilder::Handle::$db_type";
+
+    unless (eval "require $package; 1;") {
         die "Unable to load DBIx::SearchBuilder database handle for '$db_type'.\n".
             "Perhaps you've picked an invalid database type or spelled it incorrectly.\n".
             $@;
     }
 
+    @RT::Handle::ISA = ($package);
+
     # We use COLLATE NOCASE to enforce case insensitivity on the normally
     # case-sensitive SQLite, LOWER() approach works, but lucks performance
     # due to absence of functional indexes