public function testSave()
{
    $this->pageResource->expects($this->once())
        ->method('save')
        ->with($this->page)
        ->willReturnSelf();
    $this->assertEquals($this->page, $this->repository->save($this->page));
}

vendor/phpunit/phpunit/phpunit -c dev/tests/unit/phpunit.xml.dist vendor/yourVendor/yourModule/Test/Unit/

vendor/phpunit/phpunit/phpunit -c dev/tests/unit/phpunit.xml.dist app/code/yourVendor/yourModule/Test/Unit/

bin/magento dev:tests:run unit

MagentoCmsTestUnitModelPageRepositoryTest