test(cactus-core): refactor handle-rest-endpoint-exception.test.ts

[petermetz]

This change makes it easier to read and debug some of the assertions of
the test case which are a little convoluted due to the usage of the Jest
spies.

These changes were done as part of another fix but ultimately turned out
to be not a factor for that fix and therefore I thought it bess to separate
the change out onto it's own PR to make it easier to review.

Related to #2966 but this is not the actual fix.

Signed-off-by: Peter Somogyvari [email protected]

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A Must Read for Beginners
For rebasing and squashing, here's a must read guide for beginners.

[jagpreetsinghsasan]

LGTM