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/regex_test.cpp

@@ -28,7 +28,7 @@ TEST(regex, smoke) {
 
   char buf[80];
   regerror(REG_NOMATCH, &re, buf, sizeof(buf));
-#if __BIONIC__
+#if defined(__BIONIC__)
   ASSERT_STREQ("regexec() failed to match", buf);
 #else
   ASSERT_STREQ("No match", buf);