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 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

[harjothkhara]

This is a tests-only change. Could a triager please add the skip news label?

[harjothkhara]

This PR only touches Lib/test/test_embed.py — it adds a regression test for test_embed when the build uses a split exec prefix (gh-151096). There's no user-facing or behavioral change, so there's nothing to record in Misc/NEWS.d.

Could a core dev apply the skip news label so the bedevere/news check goes green? Happy to add a blurb instead if that's preferred.

[harjothkhara]

Added a Tests NEWS entry (Misc/NEWS.d/next/Tests/…gh-issue-151096…) so bedevere/news is satisfied — no skip news needed. This is purely the test fix for gh-151096; no behavioral change. Ready for review.

[harjothkhara]

@vstinner — small test-only fix in your test_embed area: test_init_is_python_build_with_home (and the module_search_paths helper) assumed exec_prefix == prefix, so test_embed fails when configured with --exec-prefix differing from --prefix (gh-151096); the fix reads sys.exec_prefix / config['exec_prefix'] separately. CI is green and there's a Tests NEWS entry — would you be able to review/merge when you have a moment? Happy to adjust.