Bugs in `create_test_image_2d` and `create_test_image_3d`

[yiheng-wang-nv]

Describe the bug

1. There is a limitation in the current implementation:
   The following line:

rad = rs.randint(5, rad_max)

requires rad_max be larger than 5.

I think we should not use the magic number 5 here, and there is no docstring about this limitation.

2. The functions should also ensure that rad_max is less than the input size.

To Reproduce

from monai.data import create_test_image_3d
im, seg = create_test_image_3d(20, 20, 20)

or

im, seg = create_test_image_3d(128, 128, 128, rad_max=5)

3. There is no unittest for these two functions.

[yiheng-wang-nv]

I will enhance these functions later to resolve the bugs.