Add rustdoc test for const computed value

Author: GuillaumeGomez

src/test/rustdoc/const-value-display.rs

@@ -0,0 +1,9 @@
+#![crate_name = "foo"]
+
+// @has 'foo/constant.HOUR_IN_SECONDS.html'
+// @has - '//*[@class="docblock item-decl"]//code' 'pub const HOUR_IN_SECONDS: u64 = 60 * 60; // 3_600u64'
+pub const HOUR_IN_SECONDS: u64 = 60 * 60;
+
+// @has 'foo/constant.NEGATIVE.html'
+// @has - '//*[@class="docblock item-decl"]//code' 'pub const NEGATIVE: i64 = -60 * 60; // -3_600i64'
+pub const NEGATIVE: i64 = -60 * 60;