gdb/testsuite: add new test for comparing char types in Python

gdb/testsuite: add new test for comparing char types in Python

There's an interesting property of the 'char' type in C and C++, the
three types 'char', 'unsigned char', and 'signed char', are all
considered distinct.

In contrast, and 'int' is signed by default, and so 'int' and 'signed
int' are considered the same type.

This commit adds a test to ensure that this edge case is visible to a
user from Python.

It is worth noting that for any particular compiler implementation (or
the flags a compiler was invoked with), a 'char' will be either signed
or unsigned; it has to be one or the other, and a user can access this
information by using the Type.is_signed property.  However, for
something like function overload resolution, the 'char' type is
considered distinct from the signed and unsigned variants.

There's no change to GDB with this commit, this is just adding a new
test to guard some existing functionality.