Extract APR and temp-fixture-file helper code to indra/test.

Specifically:
Introduce ManageAPR class in indra/test/manageapr.h. This is useful for a
simple test program without lots of static constructors.
Extract NamedTempFile from llsdserialize_test.cpp to indra/test/
namedtempfile.h. Refactor to use APR file operations rather than platform-
dependent APIs.
Use NamedTempFile for llprocesslauncher_test.cpp.