Update JS tests styleguide and CSS class naming conventions.

[ioanacristinamarinescu]

• We need to update the JS unit test style guide - https://geck.chevah.com/styleguide/javascript.html#test-styleguide as it is no longer relevant for jest.

• we should update the CSS class name convention - https://geck.chevah.com/styleguide/css.html#css-class-naming
  not to use CSS class names for JS or testing purposes. instead, we should use data-test and data-js attributes
  the CSS class name is a hack that hijacks the CSS in a way that is not needed
  it might had made sense a long time ago when there was no good support for data-* attributes

[adiroiban]

See the tests from here https://github.com/chevah/server/pull/5819/commits/84f4b4f5bd4ad66f2f0248ad3208f2dcdff35f28#diff-ee37558d8b70bb567b20f311e764d733386ed4a3a7618c94eeb1fb9efc88acf1

[ioanacristinamarinescu]

for tests https://github.com/chevah/server/pull/5840#discussion_r929935450