[Extensions] "Fix" and enable test MultipleEntryPointsExtension.MultipleEntryPoints

This test was added by commit 77bcab7bd3ba998196da6958c590af9fe32a1cd7
"[Extensions] Add tests for loading extension with multiple entry points", but
it was never enabled.

Later then, commit 76dbc259ef6e58c1667f763403201c50d4803bc5 "[Extensions] Reduce
boilerplate for external extension tests" mispelled the path for the
multiple_entry_points_extension.so.

This patch fixes this and enables this test.
Merge pull request #1477 from jeez/entry_points-fix-test

[Extensions] "Fix" and enable test MultipleEntryPointsExtension.Multiple...