gh-42477: Normalize temporary path in docbuild doctest

[cxzhong]

Summary

• compare the namespace-package import root with the canonical temporary path
• make the doctest portable to macOS, where a temporary path under /var may resolve under /private/var

Root cause

_extend_over_namespace_packages() deliberately canonicalizes both sys.path entries and the input directory with os.path.realpath(). The doctest compared the returned canonical path with the unnormalized string returned by TemporaryDirectory(). These strings can differ on macOS even though they identify the same directory.

This changes only the doctest expectation; runtime behavior is unchanged.

Follow-up to #42477.

Testing

• python src/bin/sage-runtests --warn-long 5.0 src/sage_docbuild/builders.py
  • 136 tests passed

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[jhpalmieri]

Marking as a blocker since without it there is a failing doctest.