test: stabilize HOTKEYS MULTI/EXEC test by increasing iteration count (#15129)

## Problem

The test `HOTKEYS - commands inside MULTI/EXEC` in
`tests/unit/hotkeys.tcl` is flaky on fast hardware. This PR raises its
inner loop count from 7 to 30 to make `key2` reliably appear in the CPU
top-K.

Failed CI:
https://github.com/redis/redis/actions/runs/25051455424/job/73380034469?pr=15128

Inside `MULTI`/`EXEC`, each queued command's per-command CPU time is
recorded as `c->duration = ustime() - call_timer` (microseconds,
integer). Very fast commands such as `SET` against a small value can
complete in less than 1 µs and therefore be measured as `0`.

`hotkeyStatsUpdateCurrentCmd` then forwards that zero duration as the
weight to `chkTopKUpdate`, which has an explicit early return on `weight
== 0`:

```c
sds chkTopKUpdate(chkTopK *topk, char *item, int itemlen, counter_t weight) {
    if (weight == 0) return NULL;
    ...
}
```

In the original test, `key2` is `SET` only 7 times inside the
transaction. On fast hosts (the failure was observed on an ARM box with
`ustime()` ticking at 1 µs resolution) it is possible for all 7 calls to
be measured as 0 µs, which means `key2` is never inserted into the CPU
top-K and the assertion

```tcl
assert [dict exists $cpu_result $key2]
```

fails. `key1` has 21 calls and is statistically safe.

The author already anticipated this and left a comment ("Send multiple
commands to avoid <1us cpu for $key2"), but 7 iterations turned out to
be insufficient.

## Changes

Bump the iteration count from 7 to 30. With `key2` now `SET` 30 times
the probability of every single call being measured as 0 µs becomes
negligible on any realistic hardware.

tests/unit/hotkeys.tcl

@@ -251,7 +251,7 @@ start_server {tags {external:skip "hotkeys"}} {
         r multi
         # Send multiple commands to avoid <1us cpu for $key2 which we assert
         # at end of test
-        for {set i 0} {$i < 7} {incr i} {
+        for {set i 0} {$i < 30} {incr i} {
             r set $key1 value1
             r set $key2 value1
             r set $key1 value2