HPCC-12619 Add //class=embedded into the relevant test cases

Add tag to
    - cassandra-simple.ecl
    - embedR.ecl (+banner)
    - mysqlembed.ecl

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #6671 from AttilaVamos/HPCC-12619-improvement

HPCC-12619 Add //class=embedded into the relevant test cases

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>