[NFC] Normalize some test 'main' signatures

There were 3 tests with 'int main(void)', and 6 with the return type on a different line. I'm about to send a patch for main in tests, and this NFC change is unrelated.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350770 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed