[libFuzzer] Disable print-func.test on Darwin.
The try-lock guard change seems to be making this test fail on Mac, but
I haven't been able to reproduce the failure.  Disabling the test on Mac
to fix build bot.
+UNSUPPORTED: darwin
RUN: %cpp_compiler %S/PrintFuncTest.cpp -o %t
RUN: %t -seed=1 -runs=100000 2>&1 | FileCheck %s
RUN: %t -seed=1 -runs=100000 -print_funcs=0 2>&1 | FileCheck %s --check-prefix=NO
