Fix core3.test_asyncify_lists* when Emscripten SDK was built from source.

[juj]

The self.assertFalse(b'main' in binary) check would fail because the binary does, and should, contain the name main, in an assert() check. In disassembly:

 (data $0 (i32.const 1024) "c_baz\00-+   0X0x\00c:\\emsdk\\emscripten\\main\\test\\core\\test_asyncify_lists.cpp\00foo\00a == b\00(null)\00bar: %d\n\00foo: %d\n\00c++: %d\n")

i.e. the occurrence of main is in a path name.

Change the code to check another symbol, and verify that the symbol name is caught in some other mode so it won't become tautologically true in a future refactor.