Split G4VG test into Gdml and Custom

[sethrj]

This separates the GDML loading tests from the custom tests.

[Copilot]

Pull Request Overview

This PR refactors the test suite by separating GDML loading tests from custom geometry builder tests into two distinct test files. The core functionality for testing has been extracted into a reusable TestBase class that both test suites inherit from.

Key Changes

• Created TestBase class to provide common test infrastructure for geometry conversion validation
• Split the original test file into Gdml.test.cc for GDML-based tests and Custom.test.cc for custom geometry tests
• Updated build system to compile separate test executables linked against a shared test library

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/TestBase.hh	Declares the new base test class with common test infrastructure
test/TestBase.cc	Implements shared test functionality extracted from the original test file
test/LoadGdml.hh	Removed file containing GDML loading utilities
test/Gdml.test.cc	GDML loading tests now use TestBase and inline the GDML loading logic
test/Custom.test.cc	New file containing custom geometry builder tests
test/CMakeLists.txt	Updated build configuration to create separate test executables

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