ENH: Convert six no-arg ImageIntensity tests to GoogleTest

[hjmjohnson]

Mechanical CTest → GoogleTest conversion of the six alphabetically-first no-argument tests under `Modules/Filtering/ImageIntensity/test/`. Same pattern as #6071 and #6132. All six pass against `ITKImageIntensityGTestDriver` locally.

Conversions

#	Old	New
1	`itkAbsImageFilterAndAdaptorTest`	`itkAbsImageFilterAndAdaptorGTest`
2	`itkAcosImageFilterAndAdaptorTest`	`itkAcosImageFilterAndAdaptorGTest`
3	`itkAdaptImageFilterTest`	`itkAdaptImageFilterGTest`
4	`itkAdaptImageFilterTest2`	`itkAdaptImageFilterGTest2`
5	`itkAddImageAdaptorTest`	`itkAddImageAdaptorGTest`
6	`itkAddImageFilterFrameTest`	`itkAddImageFilterFrameGTest`

Each conversion is a single `git mv`-tracked rename + body edit + `CMakeLists.txt` update — `git log --follow` keeps the pre-conversion history. CMake source-list entries kept alphabetically sorted in both `ITKImageIntensityTests` (legacy) and `ITKImageIntensityGTests`.

Mechanical-conversion details

• `int itkFooTest(int, char *[])` → `TEST(Foo, ConvertedLegacyTest)`.
• `if (...) return EXIT_FAILURE` → `EXPECT_*` (or `EXPECT_THROW` for the three exception cases in `AddImageFilterFrame`).
• `ITK_EXERCISE_BASIC_OBJECT_METHODS` → `ITK_GTEST_EXERCISE_BASIC_OBJECT_METHODS` (avoids the `return EXIT_FAILURE` path that breaks under MSVC inside a void TEST body — same gotcha as ENH: Convert four no-arg Common tests to GoogleTest #6132).
• `itk::Math::FloatAlmostEqual(a, b, 10, eps)` → `EXPECT_NEAR(a, b, eps)`.
• `itk::Math::NotExactlyEquals(a, b)` predicate flips → `EXPECT_EQ(a, b)`.
• Removed redundant `std::cout`-only diagnostics on the success path; preserved scope-explaining comments.
• Helper typedefs hoisted into anonymous `namespace { }` where they were file-scope in the original.

Local verification

```
[----------] Global test environment tear-down
[==========] 11 tests from 8 test suites ran. (1 ms total)
[ PASSED ] 11 tests.
```

Filter: full `ITKImageIntensityGTestDriver` (no `--gtest_filter`). All converted tests run; pre-existing `ArithmeticOpsTest` and `BitwiseOpsTest` GTests still pass.