Simplify TAP tests of pg_dump for connection strings

 Simplify TAP tests of pg_dump for connection strings
 
 The last set of scenarios did an initialization of nodes followed by an
 extra command to set up the authentication policy with pg_regress
 --config-auth.  This configuration step can be integrated directly using
 the option auth_extra from PostgresNode::init when initializing the
 node, saving from one extra command.  On Windows, this also restricts
 more pg_ident.conf for the SSPI user mapping by removing the entry of
 the OS user running the test, which is not needed anyway.
 
 Note that IPC::Run mishandles double quotes, hence the restore user name
 is changed to map with that.  This was already done in the test as a
 later step, but not in a consistent way, causing the switch to use
 auth_extra to fail.
 
 Found while reviewing ca129e5.
 
 Discussion: https://postgr.es/m/20190703062024.GD3084@paquier.xyz