MDEV-25884 Tests use environment $USER variable without quotes

mysql-test/suite/mariabackup/auth_plugin_win.result

 INSTALL SONAME 'auth_named_pipe';
 CREATE USER 'USERNAME' IDENTIFIED WITH named_pipe;
-GRANT ALL PRIVILEGES ON *.* to USERNAME;
+GRANT ALL PRIVILEGES ON *.* to 'USERNAME';
 DROP USER 'USERNAME';
 UNINSTALL SONAME 'auth_named_pipe';

mysql-test/suite/mariabackup/auth_plugin_win.test

@@ -18,7 +18,7 @@ INSTALL SONAME 'auth_named_pipe';
 --replace_result $USERNAME USERNAME
 eval CREATE USER '$USERNAME' IDENTIFIED WITH named_pipe;
 --replace_result $USERNAME USERNAME
-eval GRANT ALL PRIVILEGES ON *.* to $USERNAME;
+eval GRANT ALL PRIVILEGES ON *.* to '$USERNAME';
 
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 --disable_result_log

mysql-test/suite/plugins/r/unix_socket.result

@@ -2,7 +2,7 @@ install plugin unix_socket soname 'auth_socket.so';
 #
 # with named user
 #
-create user USER identified via unix_socket;
+create user 'USER' identified via unix_socket;
 #
 # name match = ok
 #
@@ -12,7 +12,7 @@ USER@localhost	USER@%	test
 #
 # name does not match = failure
 #
-drop user USER;
+drop user 'USER';
 #
 # and now with anonymous user
 #

mysql-test/suite/plugins/t/unix_socket.test

@@ -12,9 +12,9 @@ eval install plugin unix_socket soname '$AUTH_SOCKET_SO';
 --echo # with named user
 --echo #
 
---let $replace=create user $USER
---replace_result $replace "create user USER"
-eval create user $USER identified via unix_socket;
+--let $replace=create user '$USER'
+--replace_result $replace "create user 'USER'"
+eval create user '$USER' identified via unix_socket;
 
 --write_file $MYSQLTEST_VARDIR/tmp/peercred_test.txt
 --let $replace1=$USER@localhost
@@ -34,9 +34,9 @@ EOF
 --error 1
 --exec $MYSQL_TEST -u foobar --plugin-dir=$plugindir < $MYSQLTEST_VARDIR/tmp/peercred_test.txt
 
---let $replace=drop user $USER
---replace_result $replace "drop user USER"
-eval drop user $USER;
+--let $replace=drop user '$USER'
+--replace_result $replace "drop user 'USER'"
+eval drop user '$USER';
 
 --echo #
 --echo # and now with anonymous user