feat: Support for PHPUnit 10 (#56)

Require phpspec/prophecy as dev dependency (#51)

* Require phpspec/prophecy as dev dependency

sebastianbergmann/phpunit#5033 changed phpunit
to no longer depend on phpspec/prophecy. This results in the need to add
a development dependency for phpspec/prophecy. Doing so results in a
warning, since PHPUnit\Framework\TestCase::prophesize() is deprecated
and will be removed in PHPUnit 10. While we could use the trait provided
by phpspec/prophecy-phpunit instead, it would break PHP < 7.3 support.

* Closes #50

php-mock/php-mock-phpunit 2.6.0

- #45 Added PHPUnit 9 support

php-mock/php-mock-phpunit 2.5.0

- #40 adds PHPUnit 8.3 and 8.4 support

php-mock/php-mock-phpunit 2.4.0

- #37 adds PHPUnit 8.2 support
- #37 removes HHVM support

php-mock/php-mock-phpunit 2.3.0

- Adds PHPUnit 8.1 support

php-mock/php-mock-phpunit 2.2.0

- Added PHPUnit 8 support

php-mock-phpunit 2.1.2

    Added:
      - [#30](#30) adds
        PHPUnit 7.4 support

php-mock-phpunit 2.1.1

Added:
  - [#28](#28) adds
    PHPUnit 7.1 support

php-mock-phpunit 2.1.0

Added:
  - [#27](#27) adds
    PHPUnit 6.5 and 7.0 support

Removed:
  - [#27](#27) removes
    abstract methods `getMockBuilder` and `getTestResultObject` in `PHPMock` trait