Fix flaky test TestWithRsync/with_headless_tsh

[Joerger]

I updated the test failure output to include the cmd's stdout/err and found the ssh agent was causing failures.

 FAIL: TestWithRsync/with_OpenSSH_ssh (0.18s)
        proxy_test.go:489: 
                Error Trace:    /home/bjoerger/gravitational/teleport-test/tool/tsh/common/proxy_test.go:489
                Error:          Received unexpected error:
                                exit status 255
                Test:           TestWithRsync/with_OpenSSH_ssh
                Messages:       exit code: 255
                                out: sign_and_send_pubkey: signing failed for RSA "/tmp/TestWithRsync3213579311/002/keys/127.0.0.1/alice": age
nt refused operation
                                sign_and_send_pubkey: signing failed for RSA "/tmp/TestWithRsync3213579311/002/keys/127.0.0.1/alice" from agen
t: agent refused operation
                                bjoerger@127.0.0.1: Permission denied (publickey).
                                rsync: connection unexpectedly closed (0 bytes received so far) [sender]
                                rsync error: unexplained error (code 255) at io.c(231) [sender=3.2.7]

Disabling the ssh agent fixes the error, since the test uses an ssh config anyways.

Fixes #32958

[zmb3]

Thank you Brian!

[codingllama]

Thanks!

[zmb3]

@Joerger in the future you can just ask Roman or I for an exception to the flaky test detector.

Excluding tests like you've done here also requires an admin approval, so it won't save you a review and it means if the test gets faster in the future we have to remember to remove it from this list.

[public-teleport-github-review-bot]

@Joerger See the table below for backport results.

Branch	Result
branch/v12	Failed
branch/v13	Failed
branch/v14	Create PR