[DSSv3] Fix issue - 16339

[lucica28]

std.array.insertInPlace: added tests for class and union.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	16339	trivial	insertInPlace should be tested for unions and classes

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8181"

[RazvanN7]

Before this addition, the unittest tested that the code is both @safe and pure. With this addition the first paragraph no longer tests that. Ideally, the added tests should be pure and @safe; if that is not possible, then just create a new unittest block that is @System. (from looking at the code, I don't see why purity is lost; also, the class example should be @safe)