[gdb/testsuite] Record less in gdb.reverse/time-reverse.exp

[gdb/testsuite] Record less in gdb.reverse/time-reverse.exp

While stepping through gdb.reverse/time-reverse.exp I realized that we're
recording the instructions for resolving the PLT entries for functions time
and syscall, while that's not really the focus of the test-case.

Limit the scope of the test, by calling the functions once before starting
to record.

Also call "info record" after recording to make it clear how many
instructions were recorded.

On x86_64-linux, before this patch (but with info record added), we have:
...
$ grep "Log contains" gdb.log
Log contains 750 instructions.
Log contains 1218 instructions.
...
and with this patch we have:
...
$ grep "Log contains" gdb.log
Log contains 24 instructions.
Log contains 19 instructions.
...

Tested on x86_64-linux.

Approved-By: Guinevere Larsen <guinevere@redhat.com>