Well, first, you need to decide what it is you actually want to test.
There doesn't seem to be any actual functionality in your class - all it has is:
- a constructor that takes some arguments and, presumably, initialises some of the data members
- some "get" access methods for those data members.
- an increase_limit method, that, presumably, changes the value of the credit_limit data member
So, really, the only thing you can test is that, when you call those "get" methods, they give you the same values as you originally supplied in the constructor.
As written, there's no way to test the increase_limit method, because you haven't supplied any way for a test program to obtain the value of it. Did you intend to write a "get" method for that too?
EDIT: And please use code tags when posting code, to make it readable: