Model spec test

[kessie2862]

This pull request contains changes in the following files:

• reservation_spec.rb
• unit_spec.rb
• user_spec.rb
• rails_helper.rb and spec_helper.rb

Changes Made

• Add unit specs for associations and validations in reservation model.
• Add unit specs for associations and validations in unit model.
• Add unit specs for associations and validations in user model.
• Included rails_helper.rb and spec_helper.rb files to help organize and configure the testing environment.
• Added rspec-rails gem to extend the RSpec testing framework and shoulda-matchers to simplify testing common Rails conventions.