Replace existsSync() usage in tests (#156)

Use the `exists` `Matcher` instead of `existsSync()` in tests.
Mechanical change to replace:

    expect(f.existsSync(), true);
    expect(f.existsSync(), isTrue);

and

    expect(g.existsSync(), false);
    expect(g.existsSync(), isFalse);

with

    expect(f, exists);
    expect(g, isNot(exists));
2 files changed