Fix unstable test names in gdb.python/py-objfile.exp

Fix unstable test names in gdb.python/py-objfile.exp

Currently, if you diff testsuite/gdb.sum of different builds you see
this spurious hunk:

  -PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id" (6a0bfcab663f9810ccff33c756afdebb940037d4)
  +PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id" (1f5531c657c57777b05fc95baa0025fd1d115c3b)

Fix this by syncing get_python_valueof with get_integer_valueof, which
stopped outputting the value in commit 2f20e312aad6
("get_integer_valueof: Don't output value in test name").

After this commit we'll show:

  PASS: gdb.python/py-objfile.exp: get python valueof "sep_objfile.build_id"

As the comment explicitly says get_python_valueof is modeled on
get_integer_valueof, I went ahead and also added the optional 'test'
parameter while at it.

gdb/testsuite/ChangeLog:
2017-10-24  Pedro Alves  <palves@redhat.com>

	* lib/gdb-python.exp (get_python_valueof): Add 'test' optional
	parameter and handle it.  Don't output read value in test name.