Refactor to support more varied testing.

Refactor the test harness to support three types of tests:
- unit: White box unit tests.  These tests have full access to all
  internal jemalloc library symbols.  Though in actuality all symbols
  are prefixed by jet_, macro-based name mangling abstracts this away
  from test code.
- integration: Black box integration tests.  These tests link with
  the installable shared jemalloc library, and with the exception of
  some utility code and configure-generated macro definitions, they have
  no access to jemalloc internals.
- stress: Black box stress tests.  These tests link with the installable
  shared jemalloc library, as well as with an internal allocator with
  symbols prefixed by jet_ (same as for unit tests) that can be used to
  allocate data structures that are internal to the test code.

Move existing tests into test/{unit,integration}/ as appropriate.

Split out internal parts of jemalloc_defs.h.in and put them in
jemalloc_internal_defs.h.in.  This reduces internals exposure to
applications that #include <jemalloc/jemalloc.h>.

Refactor jemalloc.h header generation so that a single header file
results, and the prototypes can be used to generate jet_ prototypes for
tests.  Split jemalloc.h.in into multiple parts (jemalloc_defs.h.in,
jemalloc_macros.h.in, jemalloc_protos.h.in, jemalloc_mangle.h.in) and
use a shell script to generate a unified jemalloc.h at configure time.

Change the default private namespace prefix from "" to "je_".

Add missing private namespace mangling.

Remove hard-coded private_namespace.h.  Instead generate it and
private_unnamespace.h from private_symbols.txt.  Use similar logic for
public symbols, which aids in name mangling for jet_ symbols.

Add test_warn() and test_fail().  Replace existing exit(1) calls with
test_fail() calls.