test/helpers:h: fixed initialization order in givenACodeSampleToTokenize

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Discovered while working on #5848.

I filed https://trac.cppcheck.net/ticket/12322 about detecting this.

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It is detected by valgrind:

==128445== Conditional jump or move depends on uninitialised value(s)
==128445==    at 0x7FCB88: TokenList::TokenList(Settings const*) (tokenlist.cpp:53)
==128445==    by 0x2EE861: Tokenizer::Tokenizer(Settings const*, ErrorLogger*, Preprocessor const*) (tokenize.cpp:156)
==128445==    by 0x9E519D: givenACodeSampleToTokenize::givenACodeSampleToTokenize(char const*, bool, bool) (helpers.h:45)
==128445==    by 0x9F1CA0: TestLibrary::container() const (testlibrary.cpp:873)
==128445==    by 0x9E5A4C: TestLibrary::run() (testlibrary.cpp:70)
==128445==    by 0x856F00: TestFixture::run(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (fixture.cpp:339)
==128445==    by 0x8574BB: TestFixture::runTests(options const&) (fixture.cpp:376)
==128445==    by 0x85C99C: main (main.cpp:42)

But we do not run any tests in it which use this code so that step did not fail in the other build. We should enable more tests and only omit the really slow ones. I filed https://trac.cppcheck.net/ticket/12323 about it.