Add tests for `assert_equal_public_point`

[moCello]

Describe what you want implemented
Add tests for assert_equal_public_point method of the composer:

• sanity test
• fails when the points are not equal
• fails when only one of the coordinates are not equal

Describe "Why" this is needed
We need to increase our tests coverage, as the issue #721 clearly showed.

Describe alternatives you've considered
N/A

Additional context
Equivalent to #725

[moCello]

included in #731