Add HamcrestPHPUnitIntegration trait

Add HamcrestPHPUnitIntegration trait

Hamcrest has its own assertThat() function, which is used in PHPUnit
tests. However, in PHPUnit 6, tests that don't have any assertions are
marked as risky.

To work around that, add a HamcrestPHPUnitIntegration trait that
provides a $this->assertThatHamcrest() - it wraps around Hamcrest's
assertThat(), and increments PHPUnit's assertion counter, so using it
ensures that the test is not risky.

Change-Id: Ia4a4ec226f64ebe90d1091ffd27420a356ca76ff