selftests/powerpc: Increase timeout for vsx_signal test

On the max config P10 machine (1920 threads and 64TB) this test fails
with a timeout:

    Sending signals to all threads 10 times...!! killing vmx_signal
    !! child died by signal 15
    failure: vmx_signal

The default timeout is 120sec so increase this 3x to 360sec. With this
change the test passes on these large machines.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230307213614.2652059-1-mikey@neuling.org