subscription_test.go to not require change of $PATH to use mocked subscription-manager.

[pvoborni]

Goal:

Remove the need to use custom $PATH which points to mocked subscription manager in order to run unit tests. The benefit is that the individual tests would be easier to run, e.g. from CLI or IDEs like vscode - e.g. to run single test from the suite.

Acceptance Criteria:

• simple go test ./... is able to run all unit tests.

Tech details:

• remove https://github.com/RedHatInsights/host-metering/blob/b50fe0e380aead3ef0a9ab6f65410d59de79b95b/Makefile#L26C4-L26C4
• mocking exec command in go is not straightforward, e.g. see https://abhinavg.net/2022/05/15/hijack-testmain/