Replace GoogleTest with doctest for unit testing

[Copilot]

GoogleTest fails to compile on recent clang versions and is years out of date. Migrated to doctest, a modern single-header testing library with no external dependencies.

Changes

• Replaced GoogleTest with doctest (v2.4.12)

  • Removed UnitTest/GoogleTest/ directory (~30K lines)
  • Added UnitTest/doctest.h and doctest_main.cc
  • Updated UnitTestBase to include doctest instead of gtest

• Converted test macros

  • TEST(Suite, Name) → TEST_CASE("Suite - Name")
  • TEST_F(Fixture, Name) → TEST_CASE_FIXTURE(Fixture, "Fixture - Name")
  • Added macro compatibility layer in UnitTest.h for EXPECT_*/ASSERT_* → CHECK/REQUIRE

• Updated test fixtures

  • Converted SetUp() → constructors, TearDown() → destructors
  • Removed override keywords and ::testing::Test base class
  • Affected: TimeoutTest, KEMFieldTest, KGeoBagShapeTest, and capacitance test fixtures

• Documentation updates

  • Updated README.md and CMakeLists.txt to reference doctest

Test Status

All existing unit tests pass except 2 pre-existing failures in IntegratorsTest (unrelated to migration):

• Kommon: 13/13 (89 assertions)
• KGeoBag: 13/13 (101 assertions)
• Kasper: 59/61 (306 assertions)

Example

Before:

TEST_F(KEMFieldCubeTest, Capacitance_GaussAnalytic) {
    // test body
}

After:

TEST_CASE_FIXTURE(KEMFieldCubeTest, "KEMFieldCubeTest - Capacitance_GaussAnalytic") {
    // test body
}

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Use doctest instead of GoogleTest</issue_title> <issue_description>GoogleTest fails to compile on recent clang versions (see KATRIN-Experiment/Kassiopeia#85 ). GoogleTest currently considers to add a new dependency to the central Google base library "abseil" ( https://github.com/google/googletest/issues/2883 ). Since our fork of GoogleTest is many years out of date and integrating a new version would be a significant effort anyways, it seems reasonable to move to the dependency-less single-header testing library doctest: https://github.com/doctest/doctest</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes KATRIN-Experiment/Kassiopeia#87

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