fix: Standardize precision alias in tests to wp (fixes #1343)

[krystophny]

Summary

• Replace direct real64/dp usage in selected tests with wp alias for consistency with src/.
• Tighten imports to follow project style (use <module>, only:) and source wp from fortplot.

Changes

• test/test_streamplot_interface_color.f90: use wp from fortplot; keep advanced API imports.
• test/test_streamplot_arrows.f90: switch to explicit fortplot imports (figure, streamplot, title, savefig, wp).
• test/test_png_file_size_scaling.f90: already used wp from fortplot (no change).
• test/test_scatter_color_cycle.f90: import wp from fortplot alongside figure_t.

Rationale

• Align tests with repository guidelines:
  • Prefer use fortplot, only: wp => real64 over intrinsic wp wiring in tests.
  • Use explicit-only imports for clarity and to prevent namespace bleed.

Verification (local)
Commands run from repository root:

• make test-ci
  • Project compiled successfully; CI essential test suite completed successfully.
  • Sample excerpt: "CI essential test suite completed successfully".
• make verify-artifacts
  • Artifact verification passed.
  • Sample excerpts:
    • "[ok] symlog ylabel shows superscript three (unicode)"
    • "[ok] output/example/fortran/pcolormesh_demo/pcolormesh_basic.pdf contains RGB Image XObject"
    • "Artifact verification passed."

Artifacts

• Example outputs unchanged under output/ and output/example/fortran/. No rendering changes expected.

Notes

• Keeps functions small and focused; no behavioral changes, only style/consistency.