[java] assert setFiles uploads reach the server in SetFilesCommandTest

[yashp676]

• Java implementation of [bidi] setFiles tests assert only on the input value, never that the file was uploaded (all bindings) #18010

What does this PR do?

Adds two round-trip tests to SetFilesCommandTest. The existing four only check
that the input's value ends with the filename, which doesn't prove the file was
uploaded. The new ones submit the form and check what the upload endpoint echoed
back into the iframe, the same way UploadTest does.

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[yashp676]

@pujagani would you have time to look at this? @titusfortner pointed me your way for Java BiDi. #18025 is also open if the UploadHandler parsing change is in your area — happy to split it differently if the two would be easier to review separately.

[yashp676]

Hi @diemol and @titusfortner now that #18025 has been merged, this is the follow-up from #18010: two round-trip tests in SetFilesCommandTest that submit the form and assert the uploaded contents reach the server, the same way UploadTest does. It's test-only, 80 lines in one file.Could one of you take a look when you have a moment? Thanks!