add unit tests for diff suppress funcs

[megan07]

addresses hashicorp/terraform-provider-google#9700

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 288 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 3 files changed, 289 insertions(+), 3 deletions(-))

[ScottSuarez]

Thanks for these :D

[megan07]

/gcbrun

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 288 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 288 insertions(+), 2 deletions(-))