tests: use static command binary name to reduce startup time

According to this issue: assert-rs/assert_cmd#6.
This also potentially solves lock issue in Travis.

Author: Mingshen Sun

tests/macros.rs

@@ -16,7 +16,7 @@ macro_rules! util_name {
 #[macro_export]
 macro_rules! new_cmd {
     () => {
-        Command::main_binary().unwrap().arg(util_name!())
+        Command::new(&*::BIN_PATH).arg(util_name!())
     };
 }
 

tests/tests.rs

@@ -18,6 +18,12 @@ extern crate predicates;
 #[macro_use]
 mod macros;
 
+use std::path::PathBuf;
+use assert_cmd::cargo;
+
+lazy_static! {
+    pub static ref BIN_PATH: PathBuf = cargo::main_binary_path().unwrap();
+}
 include!("../libmesabox/src/util/build/import.rs");
 
 macro_rules! generate_fns {