This exposes the underlying exception, improving debugging:

    Fatal error: Uncaught Exception: Cannot open source device in php-src/test.php:5
    Stack trace:
    #0 php-src/test.php(5): Random\Engine\Secure->generate()
    #1 {main}

    Next RuntimeException: Random number generation failed in php-src/test.php:5
    Stack trace:
    #0 php-src/test.php(5): Random\Engine\Secure->generate()
    #1 {main}
      thrown in php-src/test.php on line 5

This exposes the underlying exception, improving debugging:

    Exception: Cannot open source device in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getInt(1, 3)
    #1 {main}

    Next RuntimeException: Random number generation failed in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getInt(1, 3)
    #1 {main}

This improves debugging, because the actual reason for the failure is available
as a previous Exception:

    DomainException: The returned string must not be empty in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getBytes(123)
    #1 {main}

    Next RuntimeException: Random number generation failed in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getBytes(123)
    #1 {main}

…rs in 50 attempts

This improves debugging, because the actual reason for the failure is available
as a previous Exception:

    RuntimeException: Failed to generate an acceptable random number in 50 attempts in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getInt(1, 3)
    #1 {main}

    Next RuntimeException: Random number generation failed in php-src/test.php:17
    Stack trace:
    #0 php-src/test.php(17): Random\Randomizer->getInt(1, 3)
    #1 {main}

Select parameters for ->getInt() that will actually lead to unsafe behavior.

If an engine fails once it will be permanently poisoned by setting
`->last_unsafe`. This is undesirable for the test, because it skews the
results.

Fix this by creating a fresh engine for each "assertion".

As we print the full stringified exception we implicitly assert the type of the
exception. No need to be overly specific in the catch block.

…mpts