Use dedicated PHPUnit assertions

[carusogabriel]

Description

Using dedicated PHPUnit assertions, we can get better error messages when debugging errors, for example:

-$this->assertTrue($xpath->query('/html/body/p[1]/ins')->length == 1); // Failing asserting that true is false
+$this->assertEquals(1, $xpath->query('/html/body/p[1]/ins')->length); // Failing asserting that variables is equal to 1

Fixes: -

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[coveralls]

Coverage remained the same at 94.838% when pulling 663fb03 on carusogabriel:phpunit into 49eb9d1 on PHPOffice:develop.