Use raw strings on gdb.python/py-xmethods.exp (and fix Python 3.8's "SyntaxWarning...

Use raw strings on gdb.python/py-xmethods.exp (and fix Python 3.8's "SyntaxWarning: invalid escape sequence")

The way unrecognized escape sequences are handled has changed in
Python 3.8: users now see a SyntaxWarning message, which will
eventually become a SyntaxError in future versions of Python:

  (gdb) source /blabla/gdb.python/py-xmethods/py-xmethods.py
  /blabla/gdb.python/py-xmethods/py-xmethods.py:204: SyntaxWarning: invalid escape seque
  nce \+
    'operator\+',
  /blabla/gdb.python/py-xmethods/py-xmethods.py:211: SyntaxWarning: invalid escape seque
  nce \+
    'operator\+\+',

One of our testcases, gdb.python/py-xmethods.exp, contains strings in
the form of "operator\+".  This is not recognized by Python, but is
still needed by the testsuite to work properly.  The solution is
simple: we just have to make sure these strings are marked as
raw (i.e, r"").  This is what this patch does.  I took the opportunity
to also convert other strings to raw, which, in two cases, allowed the
removal of an extra backslash.

I tested this using Python 3.7 and Python 3.8, and everything works
fine.

I think I could push this as obvious, but decided to send it to
gdb-patches just in case.

gdb/testsuite/ChangeLog:
2019-08-26  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.python/py-xmethods.exp: Use raw strings when passing
	arguments to SimpleXMethodMatcher.