Previously, the heap dump endpoint test asserted that the temporary
heap dump file had been deleted as soon as the client received a
response. This led to intermittent test failures as the input
stream is closed after its contents have been sent to the client,
creating a race condition between the client receiving the response
and then asserting that the file had been deleted and the server
close the input stream and deleting the temporary file.

This commit updates the test so that, after receiving the response, it
will wait for up to 5 seconds for the server to have deleted the
temporary heap dump file.