C++17 std::optional test coverage fixups

[nigels-com]

The unit tests were failing for me without these changes.

The first Argv argument should be the program name?

And use std::optional<T>::operator==

$ cmake -DCXXOPTS_CXX_STANDARD=17 -DCMAKE_BUILD_TYPE=Release --fresh .. && make clean all test
...
Running tests...
Test project /home/nigels/dev/cxxopts/build
    Start 1: options
1/3 Test #1: options ..........................   Passed    0.00 sec
    Start 2: find-package-test
2/3 Test #2: find-package-test ................   Passed    6.40 sec
    Start 3: add-subdirectory-test
3/3 Test #3: add-subdirectory-test ............   Passed    6.39 sec

100% tests passed, 0 tests failed out of 3

And

$ test/options_test '[optional]' 
Filters: [optional]
===============================================================================
All tests passed (13 assertions in 2 test cases)

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This change is