[gdb/testsuite] Fix gdb.dwarf2/dw2-using-debug-str.exp with cc-with-gdb-index

[gdb/testsuite] Fix gdb.dwarf2/dw2-using-debug-str.exp with cc-with-gdb-index

When running with target board cc-with-gdb-index, we run into:
...
(gdb) file dw2-using-debug-str-no-debug-str^M
Reading symbols from dw2-using-debug-str-no-debug-str...^M
Dwarf Error: DW_FORM_strp used without required section^M
(gdb) FAIL: gdb.dwarf2/dw2-using-debug-str.exp: file dw2-using-debug-str
...

The test expects the dwarf error, but has no matching pattern for the entire
output.

Fix this by updating the regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-02  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-using-debug-str.exp: Update regexp to match
	cc-with-gdb-index output.