Replace std::endl with \n and flush

In some places the `std::flush` was not added, as it was sufficiently
obvious that the flush semantics are not intended. There are likely
other places where the flush semantics aren't intended, but that
is a cleanup for later.

src/catch2/internal/catch_debugger.cpp

@@ -53,7 +53,7 @@
 
             size = sizeof(info);
             if( sysctl(mib, sizeof(mib) / sizeof(*mib), &info, &size, nullptr, 0) != 0 ) {
-                Catch::cerr() << "\n** Call to sysctl failed - unable to determine if debugger is active **\n" << std::endl;
+                Catch::cerr() << "\n** Call to sysctl failed - unable to determine if debugger is active **\n\n" << std::flush;
                 return false;
             }