tests/subsys/tracing: Async tracing API test fix

Test `tracing.transport.uart.async.test` was working only due sheer
serendipity: one of the tracing strings checked,
sys_trace_k_mutex_lock_exit, was never being registered on the tracing
buffer. However, the check never got to check this string, so the test
was passing.

Why wasn't it being registered? Tracing buffer size was a bit small for the
flurry of messages - increasing it solves the problem, and is what this
patch does.

Why did the check never got to the missing string? Tracing thread has a
lower priority, so when the k_sleep of test thread expired, it was
preempted. Indeed, if the sleep was changed by any number of ticks, the
test would fail. Even changing order - and thus number - of instructions
executed could make this test fail.

While at it, fixed some typos and an imprecise commentary on code.

Fixes: #84169

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>