Travis doesn't run any tests in C++17 mode

[karzhenkov]

This is essentially not PR but an issue (for now).
All tests related to C++17 are intentionally broken here, but Travis doesn't report any errors.
A possible solution was discussed earlier in #2364 (along with a fix for std::experimental namespace).

[coveralls]

Coverage remained the same at 100.0% when pulling 9493f4b on karzhenkov:disrupt-cxx17-tests into c3c574c on nlohmann:develop.

[karzhenkov]

After the test configuration is fixed, Travis will catch the intentionally broken tests.

[karzhenkov]

With the corrected test configuration, Travis runs tests in C++17 mode and reports injected errors, for example, here.
Now let's restore the intentionally broken tests to make them work as intended. We will probably see some new errors.

[karzhenkov]

To summarize the above:

commit	summary
8247a21	The original Travis configuration leaves C++17 tests not executed. The injected errors don't cause Travis to fail.
aae0e49	The corrected Travis configuration allows the intentionally broken C++17 tests to be catched . By the way, here are also catched some other errors known from #2364.
39b8d6b	If the injected errors are then removed, Travis still reports others. The remaining errors relate to clang++-7.

[karzhenkov]

With the last one-bit improvement the issue is eventually solved)
For test-conversions on clang++-7 with CXXFLAGS=-std=c++1z Travis previously reported:

18: [doctest] assertions: 294 | 294 passed | 0 failed |

Now the result is as follows:

18: [doctest] assertions: 311 | 311 passed | 0 failed |

[nlohmann]

Looks good to me.