[AIRFLOW-7000] Allow passing in env var JSON dict in task_test

[KevinYang21]

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Issue link: AIRFLOW-7000

1. Allow batch passing in env var through JSON dict
2. Set AIRFLOW_TEST_MODE for all airflow test executions

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[kaxil]

This would be very useful 👍

[kaxil]

Why do we need AIRFLOW_TEST_MODE ?

[KevinYang21]

This is more to share AIRFLOW_TEST_MODE across all executions and provide a convinient default env var for people too lazy to specify the cli param. It's the most common one that we use( to differ dev env from prod env). Do you think it would be useful?

[kaxil]

:( Travis is sad:

___________________ TestCliTasks.test_cli_test_with_env_vars ___________________
self = <tests.cli.commands.test_task_command.TestCliTasks testMethod=test_cli_test_with_env_vars>
    def test_cli_test_with_env_vars(self):
        with redirect_stdout(io.StringIO()) as stdout:
            task_command.task_test(self.parser.parse_args([
                'tasks', 'test', 'example_passing_params_via_test_command', 'env_var_test_task',
                '--env-vars', '{"foo":"bar"}', DEFAULT_DATE.isoformat()]))
>           self.assertIn('foo=bar', stdout.getvalue())
E           AssertionError: 'foo=bar' not found in "[\x1b[34m2020-03-17 05:28:39,212\x1b[0m] {\x1b[34mtest_task_view_type_check.py:\x1b[0m48} INFO\x1b[0m - class_instance type: \x1b[1m<class 'unusual_prefix_5d280a9b385120fec3c40cfe5be04e2f41b6b5e8_test_task_view_type_check.CallableClass'>\x1b[0m\x1b[0m\n"
tests/cli/commands/test_task_command.py:127: AssertionError

[KevinYang21]

@kaxil ya somehow the stdout capture in bash operator works only after the first test run... Reverted back to PythonOperator that other tests are using and we all green :D