tests/utils/utils.sh: use `[[ ... ]]` instead of `[` or `test`

tests/utils/utils.sh: use `[[ ... ]]` instead of `[` or `test`

Even if `[` and `test` are Bash built-ins for POSIX shell backward
compatibility, `[[` has more features, and since we're using it at least
at once place, it makes sense to always use it in my opinion.

Also use `==` instead of `=`, the latter only existing for POSIX shell
backward compatibility, the former being like pretty much all the other
programming languages we use.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic3268e8d544bbbdf8f013ca3ca0659c4bc699d6d