Adjusted assertDifferentTypeOfArray test to avoid intersecting incompatible string arrays

Getting one interesting failure caused by the `lowercase-string` refinement done before:

```
  There was 1 error:

  1) Psalm\Tests\AssertAnnotationTest::testValidCode with data set "assertDifferentTypeOfArray" ('<?php\n                    /*...ts[1];')
  Psalm\Exception\CodeException: DocblockTypeContradiction - src/somefile.php:21:21 - Cannot resolve types for $parts - docblock-defined type list{0: lowercase-string, 1?: lowercase-string} does not contain list{string, string}
```

Happens on this bit:

```php
           'assertDifferentTypeOfArray' => [
                'code' => '<?php
                    /**
                     * @psalm-assert list{string, string} $value
                     * @param mixed $value
                     */
                    function isStringTuple($value): void {
                        if (!is_array($value)
                            || !isset($value[0])
                            || !isset($value[1])
                            || !is_string($value[0])
                            || !is_string($value[1])
                        ) {
                            throw new \Exception("bad");
                        }
                    }

                    $s = "";

                    $parts = explode(":", $s, 2);

                    isStringTuple($parts);

                    echo $parts[0];
                    echo $parts[1];',
            ],
```

If I change this to:

```
@psalm-assert list{lowercase-string, lowercase-string} $value
```

Then everything works: I'm wondering if this error has to do with array intersections, and whether suppressing it suffices.

For now, changing the input string, so it is not a `lowercase-string`, is sufficient.

tests/AssertAnnotationTest.php

@@ -1006,7 +1006,7 @@ class AssertAnnotationTest extends TestCase
                         }
                     }
 
-                    $s = "";
+                    $s = "Hello World!";
 
                     $parts = explode(":", $s, 2);