fix: prevent reserved variable names in tests

[bshaffer]

See googleapis/google-cloud-php#8683

Tests are failing with the following error:

This is because in one of the generated tests, the $status local variable in the test is being overwritten with a variable named $status that is used to populate a required field in the request:

    public function searchAdReviewCenterAdsExceptionTest()
    {
        //...
        $status = new stdClass();
        $status->code = Code::DATA_LOSS;
        $status->details = 'internal error';
        // ...
        $status = AdReviewCenterAdStatus::AD_REVIEW_CENTER_AD_STATUS_UNSPECIFIED;
        $request = (new SearchAdReviewCenterAdsRequest())->setParent($formattedParent)->setStatus($status);
        try {
            $gapicClient->searchAdReviewCenterAds($request);
            // If the $gapicClient method call did not throw, fail the test
            $this->fail('Expected an ApiException, but no exception was thrown.');
        } catch (ApiException $ex) {
            $this->assertEquals($status->code, $ex->getCode());
            $this->assertEquals($expectedExceptionMessage, $ex->getMessage());
        }
        //...
    }

I've fixed this by passing an array of $reservedNames to the code which generates this, and prefixing those names with request_ before they're converted to camelcased. This will result in the name being $requestStatus in this case.

[bshaffer]

it worked!