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

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

When running test-case gdb.dwarf2/dw2-filename.exp with target board -readnow,
we run into:
...
FAIL: gdb.dwarf2/dw2-filename.exp: info sources
...

The normal output is:
...
(gdb) info sources^M
Source files for which symbols have been read in:^M
^M
Source files for which symbols will be read in on demand:^M
^M
src/gdb/testsuite/gdb.dwarf2/file1.txt^M
(gdb)
...
but with -readnow file1.txt appears in the "Source files for which symbols
have been read in" catagory instead, as expected.

Fix the FAIL by making the regexp match the -readnow output.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

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

	* gdb.dwarf2/dw2-filename.exp: Update regexp for -readnow.