gh-151096: Fix test_embed with split exec prefix

[harjothkhara]

Fix test_init_is_python_build_with_home for builds configured with different --prefix and --exec-prefix.

When running from the build directory on POSIX, getpath uses PREFIX for prefix/base_prefix and EXEC_PREFIX for exec_prefix/base_exec_prefix. The test assumed both match sys.prefix, which fails on a split-prefix build. This also fixes the module_search_paths() helper so its default exec_prefix comes from the expected config's exec_prefix rather than prefix.

Verified with a local split-prefix build (--prefix=… --exec-prefix=…/exec): the targeted test failed before and passes after, and full test_embed passes. Tests-only change.

Disclosure: AI-assisted; I've reviewed it.

• Issue: test_embed fails if --exec-prefix and --prefix differ #151096

[harjothkhara]

@vstinner, this is a small test-only fix in test_embed: this makes test_init_is_python_build_with_home pass with split --prefix / --exec-prefix builds. Would you mind taking a quick look?