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/prof_idump.c

@@ -1,18 +1,5 @@
 #include "test/jemalloc_test.h"
 
-const char *malloc_conf = ""
-#ifdef JEMALLOC_PROF
-    "prof:true,prof_accum:true,prof_active:false,lg_prof_sample:0"
-    ",lg_prof_interval:0"
-#  ifdef JEMALLOC_TCACHE
-    ","
-#  endif
-#endif
-#ifdef JEMALLOC_TCACHE
-    "tcache:false"
-#endif
-    ;
-
 static bool did_prof_dump_open;
 
 static int