increase BufferPtrSendOperation test coverage

[adamsitnik]

@stephentoub wrote in #38762 (comment)

What distresses me is that the tests passed with this PR. If anything, the thing to fix here is ensuring we have tests in place to cover this case. This type will be used when performing synchronous sends on a socket that previously had asynchronous operations performed on it, and a mistake in the arguments here would manifest as a bug if the send didn't complete in one call due to the kernel buffer being full, so it's certainly harder to test, but not impossible.

recently I've spent some time debugging CopyToAsync_AllDataCopied (#38747) and I think that it's a good candidate for extending to test the sync path as well

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[wfurt]

LGTM

[stephentoub]

Thanks. Did you confirm this hits that code path?

[adamsitnik]

Did you confirm this hits that code path?

Yes, but only by inspecting the code, not actually running it (I've run in on Windows, not on Linux).

[adamsitnik]

/azp run

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[adamsitnik]

The CI failure is not related to this change (#38998), merging