[compiler-rt][Fuzzer] Relax reduce_inputs.test to account for non-determinism (#182495)

I have seen that very occasionally this test is failing on a bot, with
only 3 files in the corpus. After running the test in a loop 4000+
times, I witnessed this same failure twice.

In both cases the first corpus member was some string not containing a
'F'; the second corpus member was 'F[' or 'FZ'; and the final corpus
member 'FUZ'.

In a normal run there is an intermediate corpus member 'FU.' - so this
test is failing in very rare cases where the fuzzer gets lucky and
matches 2 branch conditions in one mutation.

This patch allows the FileCheck condition to match 3 or 4 corpus files.
It may be possible for the fuzzer to reach the target in 2 files, but I
think that if that is possible, it will be exceptionally rare.

rdar://170440934

GitOrigin-RevId: ed1a1eda0ff9c5d59b13551c0447793e92d375bd