Refactor permuted backtrace test allocation.

Refactor permuted backtrace test allocation that was originally used
only by the prof_accum test, so that it can be used by other heap
profiling test binaries.

test/src/btalloc.c

@@ -0,0 +1,8 @@
+#include "test/jemalloc_test.h"
+
+void *
+btalloc(size_t size, unsigned bits)
+{
+
+	return (btalloc_0(size, bits));
+}

test/src/btalloc_0.c

@@ -0,0 +1,3 @@
+#include "test/jemalloc_test.h"
+
+btalloc_n_gen(0)

test/src/btalloc_1.c

@@ -0,0 +1,3 @@
+#include "test/jemalloc_test.h"
+
+btalloc_n_gen(1)