Removed all instances of XXX in preparation for relaxing of FIXME rule

src/test/bench/shootout-fasta-redux.rs

@@ -56,7 +56,7 @@ static HOMO_SAPIENS: [AminoAcid, ..4] = [
     AminoAcid { c: 't' as u8, p: 0.3015094502008 },
 ];
 
-// XXX: Use map().
+// FIXME: Use map().
 fn sum_and_scale(a: &'static [AminoAcid]) -> ~[AminoAcid] {
     let mut result = ~[];
     let mut p = 0f32;

src/test/bench/shootout-k-nucleotide.rs

@@ -49,7 +49,7 @@ impl Code {
         code
     }
 
-    // XXX: Inefficient.
+    // FIXME: Inefficient.
     fn unpack(&self, frame: i32) -> ~str {
         let mut key = **self;
         let mut result = ~[];

src/test/compile-fail/lint-stability.rs

@@ -22,7 +22,7 @@ mod cross_crate {
     use self::lint_stability::*;
 
     fn test() {
-        // XXX: attributes on methods are not encoded cross crate.
+        // FIXME: attributes on methods are not encoded cross crate.
         let foo = MethodTester;
 
         deprecated(); //~ ERROR use of deprecated item

src/test/run-pass/auto-ref-slice-plus-ref.rs

@@ -37,7 +37,7 @@ pub fn main() {
     (@"test").test_imm();
     (&"test").test_imm();
 
-    // XXX: Other types of mutable vecs don't currently exist
+    // FIXME: Other types of mutable vecs don't currently exist
 
     // NB: We don't do this double autoreffing for &mut self because that would
     // allow creating a mutable pointer to a temporary, which would be a source