Standardize precision alias in tests to wp

[krystophny]

Standardize test precision alias to use wp consistently (use wp => real64 from iso_fortran_env, replace real(real64) with real(wp), and _real64 literals with _wp). This aligns tests with project style used across src/.\n\nRationale:\n- Project style uses wp alias for real64.\n- A few tests were using real64 directly or dp alias, which is inconsistent.\n\nScope:\n- test/test_streamplot_interface_color.f90\n- test/test_streamplot_arrows.f90\n- test/test_png_file_size_scaling.f90\n- test/test_scatter_color_cycle.f90\n\nImpact:\n- No functional changes; type alias only.\n- Keeps code style consistent and clearer.\n\nVerification plan:\n- Run make test-ci to ensure no regressions.\n- Run grep to confirm no remaining direct real64 uses in tests (except intentional).