So far as an individual is concerned, taking Covid-19 test as well as getting vaccinated is a personal choice.
Regarding a place of employment, asking the employees rejoining after a long leave or joining afresh to take the test is a precautionary measure in the interest of all the employees. Therefore, my personal view is that such a request can neither be improper nor illegal. Of course, the cost of the test may be reimbursed by the employer. But, in the wake of contradictory news about the negative side effects after vaccination, I think the employer should not insist on vaccination.