GroupRemovalTests class was added

New class GroupRemovalTests was added. This test deletes group by index.
GroupModificationTests class was added

New class GroupModificationTests was added. Class corresponds to test,
which edits group data. Also, type method was modified. Now method
handle situation with null value in input variable.
selectByText method was modified

selectByText method was modified. Now it handles null values in input
variable.