Use MALLOC_CONF rather than malloc_conf for tests.

malloc_conf does not reliably work with MSVC, which complains of
"inconsistent dll linkage", i.e. its inability to support the
application overriding malloc_conf when dynamically linking/loading.
Work around this limitation by adding test harness support for per test
shell script sourcing, and converting all tests to use MALLOC_CONF
instead of malloc_conf.

test/unit/lg_chunk.c

@@ -1,12 +1,5 @@
 #include "test/jemalloc_test.h"
 
-/*
- * Make sure that opt.lg_chunk clamping is sufficient.  In practice, this test
- * program will fail a debug assertion during initialization and abort (rather
- * than the test soft-failing) if clamping is insufficient.
- */
-const char *malloc_conf = "lg_chunk:0";
-
 TEST_BEGIN(test_lg_chunk_clamp)
 {
 	void *p;