Improve test pragma parsing

We should take any number of `// bindgen-$whatever` flags at the top of the
file. Only considering the first three lines of the file was confusing when I
tried adding new pragmas with an empty line between the new ones and old ones.

Author: fitzgen

tests/tests.rs

@@ -75,8 +75,12 @@ fn create_bindgen_builder(header: &PathBuf) -> Result<Option<Builder>, Error> {
     // Scoop up bindgen-flags from test header
     let mut flags = Vec::with_capacity(2);
 
-    for line in reader.lines().take(3) {
+    for line in reader.lines() {
         let line = try!(line);
+        if !line.starts_with("// bindgen") {
+            continue;
+        }
+
         if line.contains("bindgen-flags: ") {
             let extra_flags = line.split("bindgen-flags: ")
                 .last()