Add --shuffle option to randomize test execution order

[tclune]

Feature Request

Add command-line options to randomize the order in which tests are executed within each test suite.

Motivation

Randomizing test execution order helps detect hidden dependencies between tests and ensures tests are properly isolated.

Proposed Implementation

Command-line options:

• --shuffle - Enable random test ordering within each suite
• --seed=N - Specify random seed for reproducibility (implies --shuffle, 0=time-based)

Behavior:

• Shuffle tests within each suite independently (preserve suite boundaries)
• Use time-based random seed if --shuffle specified without --seed
• Tests in nested suites maintain their suite grouping

Implementation plan:

1. Add CLI arguments in src/funit/FUnit.F90
2. Implement shuffle logic in src/funit/core/TestSuite.F90
3. Add unit tests in tests/funit-core/Test_TestSuite_Shuffle.F90
4. Add integration test (ctest with shell script)

Testing

• Unit tests to verify shuffle behavior, seed reproducibility, suite boundary preservation
• Integration tests via command-line execution