SERVER-14278 Additional testing for $push with $sort

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-14280 more tests for regex query with positional operator update

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-12642 validates input to the distinct command

Signed-off-by: Benety Goh <benety@mongodb.com>
Merge remote-tracking branch 'upstream/master'