[gdb/testsuite] Fix gdb.dwarf2/debug-names.exp

When running test-case gdb.dwarf2/debug-names.exp on openSUSE Tumbleweed, I
run into:
...
(gdb) maint info symtabs^M
  ...
ERROR: internal buffer is full.
UNRESOLVED: gdb.dwarf2/debug-names.exp: break _start expanded symtab
...

Fix this by simplifying the test-case to print _start rather running to it.

Tested on x86_64-linux.
1 file changed