The VS 2017 jobs hit AppVeyor's 60-minute limit per job while still
compiling the tests (77 of about 108 test targets after 58 minutes).
They pass /std:c++17 for everything anyway, so build only the C++17
variant of each test (JSON_TestStandards=17), and split the unit test
files across two jobs each with the new JSON_TestShard=<index>/<count>
option, which keeps every <count>-th test file starting at <index>.
The extra variants of single test files are built in shard 0 only.
CMAKE_OPTIONS is no longer quoted in appveyor.yml, so that it can hold
more than one option.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
8c05e40cae