test: add sample code for use BigFrames developer guide

[ashleyxuu]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/load_data_from_bigquery_test.py:17, tag bigquery_dataframes_load_data_from_bigquery
• samples/snippets/load_data_from_csv_test.py:17, tag bigquery_dataframes_load_data_from_csv
• samples/snippets/pandas_methods_test.py:17, tag bigquery_dataframes_pandas_methods
• samples/snippets/set_options_test.py:22, tag bigquery_dataframes_set_options

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[tswast]

Since we want these to appear on https://cloud.google.com/bigquery/docs/samples these code samples need to be "standalone".

According to go/code-snippets-101 and https://cloud.google.com/bigquery/docs/samples

"A standalone code snippet demonstrates just one logical action and does not need to be shown with any other code or context to be complete."

"Copy-paste-runnable. A developer should be able to copy and paste the code into their own environment and run it with as few modifications as possible."

To encourace these samples to be standalone, it's a best practice to have 1 code sample per file.