Add test for posting a join request

[hazemessam]

Issue that this pull request solves

Ref issue - # 710

Brief description of what is fixed or changed

• Removed 2 duplicated fields [domain, department] from join us validation schema
• Added test for POST /joinus

Types of changes

Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (Documentation content changed)
• Other (please describe):

Checklist

Put an x in the boxes that apply

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• My changes does not break the current system and it passes all the current test cases.

Screenshots

[hazemessam]

@Kajol-Kumari Is still any problem with this PR?

[Kajol-Kumari]

LGTM 👍