PDFBOX-5284: Refactor Refactor PDFTabulaTextStripper to improve test …

[wx930910]

Jira

• PDFBOX-5284

Description

Replace test class PDFTabulaTextStripper by spying object and improve test design

---

Motivation

• Decouple test class PDFTabulaTextStripper from production class PDFTextStripper.
• Remove the redundant test child class PDFTabulaTextStripper
• Remove the redundant constructor.

---

Key changed/added classes in this PR

• Created spying object to replace test subclass PDFTabulaTextStripper, decoupled test from production code.
• Created method that return the mocking object for code reuse.

---