[gdb/testsuite] Fix duplicate test-names in gdb.trace
In gdb.trace we have these duplicates:
...
DUPLICATE: gdb.trace/passcount.exp: 4.20a: set all three passcounts to three
DUPLICATE: gdb.trace/passcount.exp: 4.6: set passcount to zero
DUPLICATE: gdb.trace/passcount.exp: 4.7: set passcount to large number (32767)
...