pg_test_timing: fix unit typo and widen diff type

Hi,

This morning, as part of my usual routine, I synced the master branch and read through the recent commits. While reading 82c0cb4e672, I noticed a mistake in an error message. The relevant code is like:
```
diff = INSTR_TIME_GET_NANOSEC(diff_time);

fprintf(stderr, _("Time warp: %d ms\n"), diff);
```

Here, “diff" is in nanoseconds, but the error message prints ms as the unit, which is incorrect.

To fix that, I think there are two possible options:

1. Use INSTR_TIME_GET_MILLISEC to get “diff"
2. Change “ms" to “ns" in the error message

After reading through the whole file, I think option 2 is the right fix. While doing that, I also noticed another issue.

“diff" is currently defined as int32. Although one might think that is enough for a time delta, I believe it should be int64 for two reasons:

* INSTR_TIME_GET_NANOSEC() explicitly returns int64:
```
#define INSTR_TIME_GET_NANOSEC(t) \
((int64) (t).ticks)
```

* The current code has a sanity check for backward clock drift:
```
/* Did time go backwards? */
if (unlikely(diff < 0))
{
fprintf(stderr, _("Detected clock going backwards in time.\n"));
fprintf(stderr, _("Time warp: %d ms\n"), diff);
exit(1);
}
```
Clock jumping forward is also possible, and a forward jump of about 2.14 seconds would overflow int32 when expressed in nanoseconds, making the value appear negative. In that case, the code could report a “backwards” clock jump when the actual jump was forwards, which would be misleading.

To make the patch easier to process, I split it into two parts: 0001 fixes the unit in the error message, and 0002 changes the type of diff. If this gets accepted, I would be happy to squash them into one commit.

I should also note that although I noticed this while reading 82c0cb4e672, I do not think this was an oversight of that commit. More likely, because clock drift backwards is rare, this issue has simply gone unnoticed for many years.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/