Fix popen_test in MODE=dbg

ASAN and vfork() don't appear to play well together. Maybe in later
versions of GCC it'll be better. But vfork() is flirting with danger
after all and that probably doesn't make sense in ASAN mode anyway.

test/libc/stdio/popen_test.c

@@ -23,7 +23,6 @@
 TEST(popen, test) {
   int ws;
   FILE *f;
-  /* TODO(jart): look into popen() asan error */
   f = popen("echo hi", "r");
   ASSERT_NE(NULL, f);
   EXPECT_EQ('h', fgetc(f));