[gdb/testsuite] Fix duplicate test-names in gdb.trace

[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)
...

Fix these by fixing the test-names.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* gdb.trace/passcount.exp: Fix test-names.