[gdb/testsuite] Fix gdb.python/py-symbol.exp with -readnow

[gdb/testsuite] Fix gdb.python/py-symbol.exp with -readnow

When running test-case gdb.python/py-symbol.exp with target board readnow, we
get:
...
FAIL: gdb.python/py-symbol.exp: print line number of rr
FAIL: gdb.python/py-symbol.exp: print value of rr
...

These are FAILs due to PR25857.

Mark these FAILs as KFAILs.

gdb/testsuite/ChangeLog:

2020-10-28  Tom de Vries  <tdevries@suse.de>

	* gdb.python/py-symbol.exp: Add KFAILs for -readnow.