Fix bug when replacing placeholders in testdox using an associative array

[marcospassos]

At Croct we usually assign keys to data provider arguments to improve code readability:

private function dataProvider() : array
{
    return [
        '1 + 2 = 3' => [
            'augend' => 1,
            'addend' => 2,
            'result' => 3
        ]
    ];
}

It works fine in all cases except when the @testdox has placeholders. In this case you will get an error:

Undefined offset: 0

There is also a bug when omitting optional parameters in data providers:

private function dataProvider() : array
{
    return [
        [1]
    ];
}

/**
 * @testdox Message regarding $optional
 */
public function testFoo(int $value, bool $optional = true) : void
{
}

Warning:

Undefined offset: 1

This PR includes the fixes and tests for both scenarios.

[codecov]

Codecov Report

Merging #3398 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #3398      +/-   ##
============================================
+ Coverage     83.05%   83.06%   +<.01%     
- Complexity     3570     3572       +2     
============================================
  Files           143      143              
  Lines          9503     9505       +2     
============================================
+ Hits           7893     7895       +2     
  Misses         1610     1610

Impacted Files	Coverage Δ	Complexity Δ
src/Util/TestDox/NamePrettifier.php	98.8% <100%> (+0.02%)	38 <0> (+2)	⬆️

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[epdenouden]

@marcospassos Looks very useful :)

[sebastianbergmann]

Merged into 7.4, thanks.