DEFAULT_CONTROL_D_HANDLER handles a user's Control-D (^D) presses. Add
unit tests for ^D presses. Note that this commit doesn't include unit
tests for the situation when ^D is being pressed in an expression[1].
[1]:
Example of the untested situation:
% pry
[1] pry(main)> class Foo
[1] pry(main)* ^D
[2] pry(main)>
Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>
