Add pthread_exit test.

This caught a bug in the kernel where it would return Ok(24) when
passing zero extra-munmap length.

tests/Makefile

@@ -204,6 +204,7 @@ NAMES=\
 	unistd/sysconf \
 	pthread/main \
 	pthread/cleanup \
+	pthread/exit \
 	pthread/extjoin \
 	pthread/once \
 	pthread/customstack \

tests/pthread/exit.c

@@ -0,0 +1,31 @@
+#include <assert.h>
+#include <stddef.h>
+#include <unistd.h>
+
+#include <pthread.h>
+
+#include "common.h"
+
+void *routine(void *arg) {
+    assert(arg == NULL);
+
+    sleep(1);
+
+    puts("Thread ");
+
+    return NULL;
+}
+
+int main(void) {
+    int status;
+    pthread_t thread;
+
+    if ((status = pthread_create(&thread, NULL, routine, NULL)) != 0) {
+        return fail(status, "failed to create thread");
+    }
+    if ((status = pthread_detach(thread)) != 0) {
+        return fail(status, "failed to detach thread");
+    }
+
+    pthread_exit(NULL);
+}