toaster-tests: add tests for new custom image page

toaster-tests: add tests for new custom image page

Test adding a new custom image when:

1. No custom images are in the project yet.
2. User tries to add custom image which duplicates the name of
an existing custom image.
3. User tries to add custom image which duplicates the name
of a non-image recipe.

[YOCTO #9209]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>