Put exhaustive tests behind feature test

Author: gnzlbg

crates/libm-test/Cargo.toml

@@ -13,4 +13,5 @@ rand = "0.7"
 default = []
 checked = ["libm/checked"]
 stable = ["libm/stable"]
-system_libm = []
+system_libm = []
+exhaustive = []

crates/libm-test/tests/exhaustive32.rs

@@ -1,6 +1,6 @@
 //! Exhaustively test unary APIs taking 32-bit wide arguments.
 #![cfg(test)]
-#![cfg(exhaustive32)]
+#![cfg(all(exhaustive32, feature = "exhaustive"))]
 use libm_test::assert_approx_eq;
 
 macro_rules! exhaustive32 {