Fix an interaction between the oversize_threshold test and bgthds.

Also added the shared utility to check if background_thread is enabled.

test/include/test/bgthd.h

@@ -0,0 +1,17 @@
+/*
+ * Shared utility for checking if background_thread is enabled, which affects
+ * the purging behavior and assumptions in some tests.
+ */
+
+static inline bool
+is_background_thread_enabled(void) {
+	bool enabled;
+	size_t sz = sizeof(bool);
+	int ret = mallctl("background_thread", (void *)&enabled, &sz, NULL,0);
+	if (ret == ENOENT) {
+		return false;
+	}
+	assert_d_eq(ret, 0, "Unexpected mallctl error");
+
+	return enabled;
+}

test/include/test/jemalloc_test.h.in

@@ -128,6 +128,7 @@ static const bool config_debug =
 #include "test/test.h"
 #include "test/timer.h"
 #include "test/thd.h"
+#include "test/bgthd.h"
 #define MEXP 19937
 #include "test/SFMT.h"