tests: posix: pthread_pressure: enable pthread barrier in twister

In some cases, when GitHub CI is running several Qemu processes,
each with heavy loads, SMP tests have shown some flakiness. This
exhibits itself as something like

```
... pthread_pressure on qemu_riscv32 failed (Timeout)
```

That is actually not at all limited to POSIX or any particular
architecture, but is mainly to do with the host scheduler and
perhaps memory barrier operations being lost in ISA translation
by Qemu.

Collectively, we can refer to these issues as "scheduler noise".

To reduce scheduler noise in the `pthread_pressure` testsuite,
enable `CONFIG_PTHREAD_CREATE_BARRIER` in `testcase.yaml`.

Note: end-users will likely not experience the need to enable
this in `prj.conf`. E.g. the following basic test demonstrates
0 failures locallly.

```
TEST=tests/posix/pthread_pressure
twister --build-only -T $TEST &>/dev/null
FAIL=0
for ((i=0; i < 100; i++)); do
  echo "Run $((i+1))/100"
  twister --test-only -T $TEST &>/dev/null
  if [ $? -ne 0 ]; then
    FAIL=$((FAIL+1))
    echo "Failed $FAIL times"
  fi
done
echo "Failure Rate: $((FAIL))/100"
```

Signed-off-by: Christopher Friedt <cfriedt@meta.com>