gdb/testsuite: fix duplicate test names in gdb.trace/circ.exp

gdb/testsuite: fix duplicate test names in gdb.trace/circ.exp

This fixes some duplicate test names in gdb.trace/circ.exp when using
native-gdbserver and native-extended-gdbserver boards.

In this test we set the trace buffer size twice.  The same test name
was used each time the size was adjusted.

I've fixed this issue by:

  1. Creating a new proc, set_trace_buffer_size, which factors out the
  code to change the buffer size, and uses test names based on the
  size we're setting the buffer too,

  2. Calling the new proc each time we want to adjust the buffer size.

After this the duplicate test names are resolved.  There should be no
change in what is tested after this commit.