Add widget test for records table and line charts.

[hawjo01]

Also, renamed the 'components' package to 'widgets' to better reflect
what the package contains.
The line chart is somewhat weak as it only tests if some of the expected
axis labels exist, but they should only show if data is processed
successfully.

[github-actions]

LCOV of commit 5cbedc6 during Analyze and Test on Pull Request #25

Summary coverage rate:
  lines......: 43.3% (177 of 409 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                          |Lines       |Functions  |Branches    
  Filename                                |Rate     Num|Rate    Num|Rate     Num
  ==============================================================================
  lib/history_chart_screen.dart           |    -      0|    -     0|    -      0
  lib/record_input_screen.dart            |    -      0|    -     0|    -      0
  lib/widgets/date_picker_input_field.dart| 0.0%     27|    -     0|    -      0
  lib/widgets/month_dropdown_menu.dart    | 0.0%     16|    -     0|    -      0
  lib/widgets/numeric_input_field.dart    | 0.0%     17|    -     0|    -      0
  lib/widgets/records_line_chart.dart     | 0.0%     38|    -     0|    -      0
  lib/widgets/records_table.dart          | 0.0%     12|    -     0|    -      0
  lib/widgets/time_picker_input_field.dart| 0.0%     24|    -     0|    -      0
  lib/widgets/year_dropdown_menu.dart     | 0.0%     12|    -     0|    -      0