Use CTRL+a + DELETE for TextInput clear method

[tpapaioa]

Fixes #226

TextInput.fill() should clear any existing text from the element before adding the new text value. It does this by calling Selenium's WebElement.clear() method. This doesn't work reliably when the textarea element is controlled by React.

This PR updates TextInput.fill() to use keyboard actions (Control+A then Delete) to clear the field instead.

Verified in pytest:

$  pytest -k test_simple_input_send_keys_clear
=== test session starts ===
[...]
collected 169 items / 168 deselected / 1 selected                                                                                                                                                                                            

testing/test_browser.py::test_simple_input_send_keys_clear PASSED   

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.44%. Comparing base (5f54ea6) to head (0cf7111).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   86.49%   86.44%   -0.05%     
==========================================
  Files          18       18              
  Lines        2569     2567       -2     
==========================================
- Hits         2222     2219       -3     
- Misses        347      348       +1     

Flag	Coverage Δ
unittests	86.44% <100.00%> (-0.05%)	⬇️

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