[gdb/testsuite] Fix auto-indent in gdb.gdb/python-helper.exp

[gdb/testsuite] Fix auto-indent in gdb.gdb/python-helper.exp

When editing gdb.gdb/python-helper.exp, auto-indent is broken in my editor
(emacs).

The problem is that this:
...
if { 1 } {
    foo "{" "}"<ENTER>bar
}
...
produces this:
...
if { 1 } {
    foo "{" "}"
bar
}
...

Note that this doesn't happen for "{}".

Fix this by using "\{" and "\}".

Tested on x86_64-linux.