Tests String::checkPlain().
@dataProvider providerCheckPlain
string $text: The text to provide to String::checkPlain().
string $expected: The expected output from the function.
string $message: The message to provide as output for the test.
bool $ignorewarnings: Whether or not to ignore PHP 5.3+ invalid multibyte sequence warnings.
function testCheckPlain($text, $expected, $message, $ignorewarnings = FALSE) {
$result = $ignorewarnings ? @String::checkPlain($text) : String::checkPlain($text);
$this
->assertEquals($expected, $result, $message);
}