[gdb/testsuite] Improve class name in gdb.dwarf2/self-spec.exp

[gdb/testsuite] Improve class name in gdb.dwarf2/self-spec.exp

I ran into:
...
(gdb) pipe maint print objfiles self-spec | grep c1^M
    name:       c1^M
    canonical:  c1^M
    qualified:  c1^M
    [3] ((addrmap *) 0xfffedfc1f010)^M
(gdb) FAIL: gdb.dwarf2/self-spec.exp: class c1 in cooked index
...

Fix this by renaming the class from c1 to class1.

Tested on aarch64-linux.