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.

gdb/testsuite/gdb.base/style.exp

@@ -444,7 +444,7 @@ proc test_disassembler_error_handling { } {
 		 "def replacement_colorize_disasm(content,gdbarch):" \
 		 "  return None" \
 		 "gdb.styling.colorize_disasm = replacement_colorize_disasm" \
-		 "\004"] \
+		 "end"] \
 	    "setup replacement colorize_disasm function" \
 	    true
 

gdb/testsuite/gdb.python/py-arch-reg-groups.exp

@@ -102,5 +102,5 @@ gdb_py_test_silent_cmd \
 	 "    raise gdb.GdbError (\"miss-matched names\")" \
 	 "  if (r1 != r2):" \
 	 "    raise gdb.GdbError (\"miss-matched objects\")" \
-	 "\004" ] \
+	 "end" ] \
     "check names and objects match" 1

gdb/testsuite/gdb.python/py-arch-reg-names.exp

@@ -102,7 +102,7 @@ gdb_py_test_silent_cmd \
 	 "    raise gdb.GdbError (\"miss-matched names\")" \
 	 "  if (r1 != r2):" \
 	 "    raise gdb.GdbError (\"miss-matched objects\")" \
-	 "\004" ] \
+	 "end" ] \
     "check names and objects match" 1
 
 # Ensure that the '.find' method on the iterator returns the same