gdb/testsuite: use 'end' at the end of python blocks

Within the testsuite, use the keyword 'end' to terminate blocks of
Python code being sent to GDB, rather than sending \004.  I could only
find three instances of this, all in tests that I originally wrote.  I
have no memory of there being any special reason why I used \004
instead of 'end' - I assume I copied this from somewhere else that has
since changed.

Non of the tests being changed here are specifically about whether
\004 can be used to terminate a Python block, so I think switching to
the more standard 'end' keyword is the right choice.