Add tests for issue 20623 - std.conv.to!string error when argument is an inout float

[MoonlightSentinel]

This fixes the regression for the upcoming release but it the should probably reconsidered how inout behaves in heavy templated code

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	20623	regression	std.conv.to!string error when argument is an inout float

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#7413"

[MoonlightSentinel]

This will be fixed as a side effect of #7407. We should probably cherry-pick/merge that commit to stable and only add the test case.

[MoonlightSentinel]

Rebased to master since this didn't make the cut for 2.091.1