fuzzer/strncmp.test: Increase iteration to 20M.

I saw cases that this finised before finding `BINGO`, possibly
insufficient number of iteration. In my case, 11,067,133 satisfied.

So, increase the number for now. This change may increase the duration
of this in failing (`BINGO` not found) case.

Author: chapuni

compiler-rt/test/fuzzer/strncmp.test

@@ -1,4 +1,4 @@
 UNSUPPORTED: target={{.*freebsd.*}}
 RUN: %cpp_compiler %S/StrncmpTest.cpp -o %t-StrncmpTest
-RUN: not %run %t-StrncmpTest              -seed=2 -runs=10000000   2>&1 | FileCheck %s
+RUN: not %run %t-StrncmpTest              -seed=2 -runs=20000000   2>&1 | FileCheck %s
 CHECK: BINGO