[gdb/testsuite] Fix dw2-ranges-psym.exp with -readnow

[gdb/testsuite] Fix dw2-ranges-psym.exp with -readnow

When running test-case gdb.dwarf2/dw2-ranges-psym.exp with target board
-readnow, I run into:
...
(gdb) file dw2-ranges-psym^M
Reading symbols from dw2-ranges-psym...^M
Expanding full symbols from dw2-ranges-psym...^M
(gdb) set complaints 0^M
(gdb) FAIL: gdb.dwarf2/dw2-ranges-psym.exp: No complaints
...

The problem is that the regexp expects a gdb prompt immediately after the
"Reading symbols" line.

Fix this by updating the regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-22  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_load_no_complaints): Update regexp to allow
	"Expanding full symbols" Line.