selftests/powerpc: Bump up rlimit for perf-hwbreak test

The systemwide perf hardware breakpoint test tries to open a perf event
on each cpu. On large systems, we run out of file descriptors and fail
the test. Instead, have the test set the file descriptor limit to an
arbitraty high value.

Reported-by: Rohan Deshpande <rohan_d@linux.vnet.ibm.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/187fed5843cecc1e5066677b6296ee88337d7bef.1669096083.git.naveen.n.rao@linux.vnet.ibm.com