[gdb/testsuite] Avoid large timeout in gdb.base/checkpoint.exp

[gdb/testsuite] Avoid large timeout in gdb.base/checkpoint.exp

I ran the testsuite in an environment simulating a stressed system, and the
only test-cases that timed out in gdb.base were gdb.base/checkpoint.exp and
gdb.base/checkpoint-ns.exp (which includes gdb.base/checkpoints.exp).

In test-case gdb.base/checkpoint.exp there's a part where the timeout is
increased with 120 seconds (in the default case that's from 10 to 130), to
accommodate for a single command creating 600+ checkpoints.

Instead, rewrite the test to present a gdb prompt each time a checkpoint is
created, for which the default timeout is sufficient.

Also ensure that the amount of checkpoints added is exactly 600 rather than
600+.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>