Remove the Optional --garden Flag

Since it is not allowed to run a veggies test with a garden driver (and vice versa), the parser should detect which framework is used in the test files. This simplifies the UI of cart and prevents errors such as:

test_main.f90                          failed.
[  99%]Compiling...
test/test_main.f90:20:30:

   20 |         individual_tests(1) = is_leap_year_is_leap_year()
      |                              1
Error: Cannot convert TYPE(test_item_t) to TYPE(test_item_t) at (1)
<ERROR> Compilation failed for object " test_test_main.f90.o "
<ERROR>stopping due to failed compilation
STOP 1