MDEV-7089: Test failures in main.failed_auth_unixsocket and plugins.unix_socket depending on environment

The test cases had some --replace_result $USER USER. The problem is that the
value of $USER can be anything, depending on the name of the unix account that
runs the test suite. So random parts of the result can be errorneously
replaced, causing test failures.

Fix by making the replacements more specific, so they will match only the
intended stuff regardless of the value of $USER.