Added setSnapshotId method to easily override the snapshot identifier

[buismaarten]

In my project I have the following:

    #[Test]
    #[TestWith(['name' => 'dummy_name'])]
    public function testCompile(string $name): void
    {
        $result = [... retrieve file based on $name ...];

        $this->setSnapshotId($name);
        $this->assertMatchesSnapshot($result);
    }

This way it is possible to store the snapshot file using the same filename (without extension), to easily find the snapshot file based on the input file. At this moment I need to add extra code for this to make it work.

Example:

• fixtures/dummy_name.txt
• snapshots/dummy_name.yml

Let me know if unit tests are needed for this.