[php-flight] fix: use static PHPUnit assertions (#21253)

Static analysis tools such as PHPStan report errors when dynamic calls are used
for static methods.

```
ERROR Dynamic call to static method PHPUnit\Framework\Assert::assertEquals().
```

According to the source code of PHPUnit
(https://github.com/sebastianbergmann/phpunit/blob/9.5.0/src/Framework/Assert.php)
the function is indeed static.

```php
public static function assertTrue($condition, string $message = ''): void
```

This change updates to PHP Flight test template `register_routes_test.mustache`
to use static calls for PHPUnit assertions.

Author: jrenggli

modules/openapi-generator/src/main/resources/php-flight/register_routes_test.mustache

@@ -26,7 +26,7 @@ class RegisterRoutesTest extends \PHPUnit\Framework\TestCase {
         {{/operations}}
         };
         \{{invokerPackage}}\RegisterRoutes::registerRoutes($handler);
-        $this->assertTrue(true);
+        self::assertTrue(true);
     }
 {{/apis}}
 {{#models}}
@@ -41,7 +41,7 @@ class RegisterRoutesTest extends \PHPUnit\Framework\TestCase {
         {{/-first}}
         {{/enumVars}}
         {{/allowableValues}}
-        $this->assertEquals(
+        self::assertEquals(
             parseParam($value, '\\{{escapedModelPackage}}\\{{classname}}'),
             \{{modelPackage}}\{{classname}}::{{#allowableValues}}{{#enumVars}}{{#-first}}{{{name}}}{{/-first}}{{/enumVars}}{{/allowableValues}}
         );

samples/server/petstore/php-flight/Test/RegisterRoutesTest.php

@@ -29,7 +29,7 @@ public function addPet(\OpenAPIServer\Model\Pet $pet): \OpenAPIServer\Model\Pet|
             }
         };
         \OpenAPIServer\RegisterRoutes::registerRoutes($handler);
-        $this->assertTrue(true);
+        self::assertTrue(true);
     }
     public function testRegisterRoutesAbstractStoreApi(): void
     {
@@ -39,7 +39,7 @@ public function deleteOrder(string $orderId): void
             }
         };
         \OpenAPIServer\RegisterRoutes::registerRoutes($handler);
-        $this->assertTrue(true);
+        self::assertTrue(true);
     }
     public function testRegisterRoutesAbstractUserApi(): void
     {
@@ -49,28 +49,28 @@ public function createUser(\OpenAPIServer\Model\User $user): void
             }
         };
         \OpenAPIServer\RegisterRoutes::registerRoutes($handler);
-        $this->assertTrue(true);
+        self::assertTrue(true);
     }
     public function testParseParamsEnumFindPetsByStatusStatusParameterInner(): void
     {
         $value = 'available';
-        $this->assertEquals(
+        self::assertEquals(
             parseParam($value, '\\OpenAPIServer\\Model\\FindPetsByStatusStatusParameterInner'),
             \OpenAPIServer\Model\FindPetsByStatusStatusParameterInner::AVAILABLE
         );
     }
     public function testParseParamsEnumOrderStatus(): void
     {
         $value = 'placed';
-        $this->assertEquals(
+        self::assertEquals(
             parseParam($value, '\\OpenAPIServer\\Model\\OrderStatus'),
             \OpenAPIServer\Model\OrderStatus::PLACED
         );
     }
     public function testParseParamsEnumPetStatus(): void
     {
         $value = 'available';
-        $this->assertEquals(
+        self::assertEquals(
             parseParam($value, '\\OpenAPIServer\\Model\\PetStatus'),
             \OpenAPIServer\Model\PetStatus::AVAILABLE
         );