Provide documentation for running tests locally

[MisterJimson]

Attempting to run tests locally fail.

This seems to be because the tests require a docker container running as well as an .env file that configures the tests. We should provide documentation to help contributors run tests locally.

Test output without any changes after cloning:

dart test
Building package executable... (4.1s)
Built test:test.
00:01 +0: loading test/provider_test.dart                                                                                                                                                                        [dotenv] Load failed: file not found: File: '.env'
00:02 +3 -1: test/provider_test.dart: (setUpAll) [E]                                                                                                                                                             
  Null check operator used on a null value
  test/provider_test.dart 23:23  main.<fn>
  
00:02 +3 -2: test/client_test.dart: client signUp() [E]                                                                                                                                                          
  Expected: null
    Actual: 'SocketException: OS Error: Connection refused, errno = 61, address = localhost, port = 63276'
  
  package:test_api            expect
  test/client_test.dart 52:7  main.<fn>.<fn>
  
00:02 +3 -3: test/client_test.dart: client signIn() [E]                                                                                                                                                          
  Expected: null
    Actual: 'SocketException: OS Error: Connection refused, errno = 61, address = localhost, port = 63278'
  
  package:test_api            expect
  test/client_test.dart 63:7  main.<fn>.<fn>
  
00:02 +3 -4: test/client_test.dart: client Get user [E]                                                                                                                                                          
  Expected: <Instance of 'String'>
    Actual: <null>
     Which: is not an instance of 'String'
  
  package:test_api             expect
  test/client_test.dart 76:7   main.<fn>.<fn>
  test/client_test.dart 74:22  main.<fn>.<fn>
  
00:02 +3 -5: test/client_test.dart: client Set auth [E]                                                                                                                                                          
  Expected: non-empty
    Actual: ''
  
  package:test_api             expect
  test/client_test.dart 82:7   main.<fn>.<fn>
  test/client_test.dart 80:22  main.<fn>.<fn>
  
00:02 +3 -6: test/client_test.dart: client Set session [E]                                                                                                                                                       
  Expected: non-empty
    Actual: ''
  
  package:test_api             expect
  test/client_test.dart 93:7   main.<fn>.<fn>
  test/client_test.dart 91:25  main.<fn>.<fn>
  
00:02 +3 -7: test/client_test.dart: client Update user [E]                                                                                                                                                       
  Expected: null
    Actual: 'Not logged in.'
  
  package:test_api             expect
  test/client_test.dart 113:7  main.<fn>.<fn>
  
00:02 +3 -8: test/client_test.dart: client Get user after updating [E]                                                                                                                                           
  Expected: <Instance of 'String'>
    Actual: <null>
     Which: is not an instance of 'String'
  
  package:test_api              expect
  test/client_test.dart 120:7   main.<fn>.<fn>
  test/client_test.dart 118:37  main.<fn>.<fn>
  
00:02 +8 -8: Some tests failed.