Remove regex and serde tests from test coverage.

tests/regex.rs

@@ -1,3 +1,4 @@
+#![cfg(not(tarpaulin_include))]
 #![cfg(feature = "regex_support")]
 
 use evalexpr::*;

tests/serde.rs

@@ -1,3 +1,4 @@
+#![cfg(not(tarpaulin_include))]
 #![cfg(feature = "serde")]
 
 use evalexpr::{build_operator_tree, Node};