Use the following method from the sample code to create an user with unique username in test class in Salesforce.

Sample Code:

public static User createTestempUser(
	Id roleId, Id profID, String fName, String lName
) {  
  
	String orgId = UserInfo.getOrganizationId();  
	String dateString =   
	String.valueof(
		Datetime.now()
	).replace(
		' ',''
	).replace(
		':',''
	).replace(
		'-',''
	);  
	Integer randomInt = Integer.valueOf(
		math.rint(
			math.random()*1000000
		)
	);  
	String uniqueName = orgId + dateString + randomInt;  
	User tempUser = new User(  
		FirstName = fName,  
		LastName = lName,  
		email = uniqueName + '@test' + orgId + '.org',  
		Username = uniqueName + '@test' + orgId + '.org',  
		EmailEncodingKey = 'ISO-8859-1',  
		Alias = uniqueName.substring(
			18, 23
		),  
		TimeZoneSidKey = 'America/Los_Angeles',  
		LocaleSidKey = 'en_US',  
		LanguageLocaleKey = 'en_US',  
		ProfileId = profId,  
		UserRoleId = roleId 
	);  
	insert tempUser;  
	return tempUser;  
	  
}