Windows is too scrupulous for non closed file resources.
Our `RemoteFileTestSupport` recreates a test directory with files
for each test.
When we don't close some file handler, we are not able to delete the
directory and we fail with subsequent tests.
* Close `InputStream` for each polled file in new tests in the
`SftpStreamingMessageSourceTests`
* Close `CLOSEABLE_RESOURCE` in new tests in the
`FtpStreamingMessageSourceTests`
* Rework `FtpStreamingMessageSourceTests.testAllContents()` do not
poll all the files on each polling cycle - this causes a race condition
when we don't close `CLOSEABLE_RESOURCE` yet in the `StreamTransformer`,
but try to proceed with recreation a test directory structure
**Cherry-pick to 5.0.x**
6ecd948a32