userDefinedMappingsSecureByDefault in
SampleSessionWebFluxApplicationTests may fail with an NPE if the
session times out before the initial response is sent due to the absence
of a SESSION cookie in the response. The test uses a short session
timeout (2 seconds) and we've seen the problem occur on CI when,
presumably, a noisy neighbour has caused some CPU starvation.

This commit increases the timeout to 5 seconds to reduce the chance of
a short period of CPU starvation causing the test to fail.

Closes gh-25572