The -o= seems to work fine in 3.x but in 2.x it actually includes the equals sign in the path, resulting in "/=/tmp/tmp.XXXXXX". The mongodump command docs for neither 2.x nor 3.x list the equals sign as part of the -o or --out parameter. It seems to work fine in 3.x with or without the equals sign.
Updated test to reflect change to -o
3d30b6ca35