Split tests/P1206R7_ranges_to into four parts

[CaseyCarter]

,... to reduce resource consumption. The obvious approach here is to split the three big pieces (the sequence_instantiator, mappish_instantiator, and settish_instantiator tests) each into their own tests, leaving behind the three small pieces (test_reservable, test_common_constructible, and test_nested_range).

Fun fact: there are now 50 P1206R7 tests in the std suite.

[CaseyCarter]

I think this was purely code-shifting, so diffing each of the four new tests with the old P1206R7_ranges_to should "line up" the matching chunks and make for quick review.

I did also cleanup the includes a bit, adding <cstddef> for std::size_t but mostly trimming excess - notably <format> and <iostream> were handy during initial development but are no longer used by these tests.

[StephanTLavavej]

Diffing against the original test was indeed very easy, thanks for avoiding other code movement! I pushed changes to fix a stealth merge conflict and one moved test line that was revealed by the diff.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I've pushed a conflict-free merge to resolve the conflict with #3015 (git's "ort" strategy is apparently better than GitHub's strategy, possibly the old "recursive").

[StephanTLavavej]

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