[DOCS] Document BigQuery test dataset configuration

[jdimatteo]

Changes proposed in this pull request:

• document the dataset environment variable that needs to be set to run locally the BigQuery tests added in [FEATURE] Enable Expectations tests for BigQuery #3219

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[jdimatteo]

@Shinnnyshinshin can you please review this when you get a chance? This little documentation piece got missed when integrating our work in #3219.

[Shinnnyshinshin]

Thank you very much for this @jdimatteo. Looks great