Consistently use #if defined(__BIONIC__) in tests.

I've also switched some tests to be positive rather than negative,
because !defined is slightly harder to reason about and there are
only two cases: bionic and glibc.

Change-Id: I8d3ac40420ca5aead3e88c69cf293f267273c8ef

tests/stdlib_test.cpp

@@ -42,7 +42,7 @@ TEST(stdlib, lrand48_random_rand) {
   EXPECT_EQ(902267124, lrand48());
   EXPECT_EQ(132366131, lrand48());
 
-#if __BIONIC__
+#if defined(__BIONIC__)
   // On bionic, random(3) is equivalent to lrand48...
   srandom(0x01020304);
   EXPECT_EQ(1409163720, random());