[gdb/testsuite] Fix gdb.dwarf2/dw2-error.exp with -readnow

[gdb/testsuite] Fix gdb.dwarf2/dw2-error.exp with -readnow

With test-case gdb.dwarf2/dw2-error.exp and target board readnow, we get:
...
FAIL: gdb.dwarf2/dw2-error.exp: break -q main
...

In the normal case, after running into the dwarf error, the minimal symbols
are still available, but with -readnow this is not the case.

Mark the FAIL as KFAIL.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

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

	* gdb.dwarf2/dw2-error.exp: Mark failure break in main as known with
	-readnow.