ewaol-tests: Ensure failing environment clean-up results in a failed test suite

Previously, if an environment clean-up failure occured it would only be reported
to the log, while the test suite might still PASS.

To avoid silent failures and therefore potentially hidden bugs, this patch
ensures the result of the environment clean-up is reflected in the result of the
test suite.

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I8a3eda61ae8bc82a7ea5f04a7bdd8f7bfda2133c