Disable some tests if "rand" feature is disabled

This avoids some misleading errors for `cargo test`.

src/geometry/quaternion_construction.rs

@@ -891,6 +891,7 @@ where
 }
 
 #[cfg(test)]
+#[cfg(feature = "rand")]
 mod tests {
     extern crate rand_xorshift;
     use super::*;

src/linalg/symmetric_eigen.rs

@@ -337,6 +337,7 @@ mod test {
         }
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn wilkinson_shift_random() {
         for _ in 0..1000 {