chore: Add .gitattributes to ignore Jsonnet test code for linguist

[terrytangyuan]

Jsonnet is recognized as the main language of this repo but it's only for testing and shouldn't be included as part of the classification.

Reference: https://github.com/github/linguist/blob/master/docs/overrides.md#generated-code

Signed-off-by: terrytangyuan terrytangyuan@gmail.com

[gaocegege]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege, terrytangyuan

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[coveralls]

Pull Request Test Coverage Report for Build 1451591201

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 8.15%

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Totals
Change from base Build 1441186837:	0.0%
Covered Lines:	311
Relevant Lines:	3816

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💛 - Coveralls