Added Extra Tests for Splay Tree Operations

[Ramy-Badr-Ahmed]

Addendum to my #168,

• Added tests to verify the correct tree size after deleting single and multiple nodes.

  • testDeleteExistingKey(): Checks the tree size after single node deletion.
  • testDeleteMultipleKeys(): Checks the tree size after the multiple random node deletion.
  • testLargeTree(): Checks tree operations, search, delete, abd insert on large tree structure.

• Added abstract declaration for setRoot() in the SplayTreeRotations.php as it references its child class implementation as static::setRoot() in both rotateLeft() and rotateRight()

• Minor typo fixes