MDEV-27289: mtr test for WITH_SERVER_EMBEDDED=ON reenable

mtr is checking the wrong path for the embedded executable
on out of tree builds.

The is_embedded.inc tests are also checking the version rather
than the MTR MYSQL_EMBEDDED environment variable.

As a result, a few tests are out of date in the result recordings.

mysql-test/include/is_embedded.inc

@@ -1,4 +1,4 @@
-if (`SELECT VERSION() NOT LIKE '%embedded%'`)
+if(!$MYSQL_EMBEDDED)
 {
    --skip Test requires: embedded server
 }