test: add ut for encryptInTransitField

[cvvz]

What type of PR is this?
/kind test

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[andyzhangx]

could you also add a negative case?

[k8s-ci-robot]

@cvvz: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-azurefile-csi-driver-unit	2339bd1	link	true	/test pull-azurefile-csi-driver-unit
pull-azurefile-csi-driver-verify	2339bd1	link	true	/test pull-azurefile-csi-driver-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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