context consistency: switch Background() to TODO()

[sebrandon1]

Make the usage of context.Background() and context.TODO() the same throughout the repo.

There were more references of TODO than Background.

Similar to:

• https://github.com/test-network-function/cnf-certification-test/pull/1216
• https://github.com/test-network-function/cnfcert-tests-verification/pull/398