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 expected both values to match sys.prefix, which fails for split-prefix builds.

This also updates module_search_paths() so its default exec_prefix comes from the expected config’s exec_prefix instead of prefix.

Validation:

• Configured a local split-prefix build with --prefix=/private/tmp/cpython-151096-install --exec-prefix=/private/tmp/cpython-151096-install/exec.
• Before the fix, ./python.exe -m test test_embed -v -m test_init_is_python_build_with_home failed with mismatched exec_prefix / base_exec_prefix.
• After the fix, ./python.exe -m test test_embed -v -m test_init_is_python_build_with_home passes.
• Full ./python.exe -m test test_embed -v passes.

This is a tests-only change; skip news.

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