Ignore slow tests when running in miri.

There's a few tests that do millions of iterations. Miri is incredibly
slow and takes too long to run these (I gave up waiting after 5+ minutes),
so we ignore them when testing with miri.

src/spsc.rs

@@ -621,6 +621,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg_attr(miri, ignore)] // too slow
     fn len() {
         let mut rb: Queue<i32, 3> = Queue::new();
 
@@ -641,6 +642,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg_attr(miri, ignore)] // too slow
     fn try_overflow() {
         const N: usize = 23;
         let mut rb: Queue<i32, N> = Queue::new();

tests/tsan.rs

@@ -74,6 +74,7 @@ fn scoped() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // too slow
 fn contention() {
     const N: usize = 1024;
 
@@ -118,6 +119,7 @@ fn contention() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // too slow
 fn mpmc_contention() {
     const N: u32 = 64;
 
@@ -163,6 +165,7 @@ fn mpmc_contention() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // too slow
 fn unchecked() {
     const N: usize = 1024;