ftrace:test.d--event--subsystem-enable.tc fails with timeout on focal:linux-hwe-5.15 ARM64

First time seen this since ftrace refactor out from ubuntu_kernel_selftests to ubuntu-kselftests-ftrace

Issue found on ARM64 nodes with both generic / generic-64k kernel.

Partial test log:
 Running './ftracetest -vvv test.d/event/subsystem-enable.tc'
 === Ftrace unit tests ===
 ....
 [1] event tracing - enable/disable with event level files
 + [ 2 -eq 1 ]
 + [ -f set_event_pid ]
 + echo
 + [ -f set_ftrace_pid ]
 + echo
 + [ -f set_ftrace_notrace ]
 + echo
 + [ -f set_graph_function ]
 + echo
 + tee set_graph_function set_graph_notrace
 + [ -f stack_trace_filter ]
 + echo
 + [ -f kprobe_events ]
 + echo
 + [ -f uprobe_events ]
 + echo
 + [ -f synthetic_events ]
 + echo
 + [ -f snapshot ]
 + echo 0
 + clear_trace
 + echo
 + enable_tracing
 + echo 1
 + . /home/ubuntu/autotest/client/tmp/ubuntu_kselftests_ftrace/src/linux/tools/testing/selftests/ftrace/test.d/event/event-enable.tc
 + echo sched:sched_switch
 + yield
 + ping 127.0.0.1 -c 1
 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.096 ms

 --- 127.0.0.1 ping statistics ---
 1 packets transmitted, 1 received, 0% packet loss, time 0ms
 rtt min/avg/max/mdev = 0.096/0.096/0.096/0.000 ms
 + cat trace
 + grep sched_switch
 + wc -l
 Timer expired (600 sec.), nuking pid 20982

From the code (tools/testing/selftests/ftrace/test.d/event/event-enable.tc) this test is trying to run:

    count=`cat trace | grep sched_switch | wc -l`

This command caused the timeout as `cat trace` seems to be never ending.