Use bash as shell for all integration tests (#479)

This commit changes the integration tests to run with bash
as the shell, instead of bash, dash, and the system's sh.

Running the integration tests with multiple shells has never
revealed a bug in Just, and they make the tests take longer,
so let's get rid of them.

tests/integration.rs

@@ -38,21 +38,14 @@ macro_rules! integration_test {
     stderr:   $stderr:expr,
     status:   $status:expr,
   ) => {
-    mod $name {
-      use super::*;
-
-      // silence unused import warnings
-      const __: i32 = EXIT_SUCCESS;
-
-      #[test] fn sh()   { integration_test("sh",   $text, &[$($arg)*], $stdin, $stdout, $stderr, $status); }
-      #[test] fn dash() { integration_test("dash", $text, &[$($arg)*], $stdin, $stdout, $stderr, $status); }
-      #[test] fn bash() { integration_test("bash", $text, &[$($arg)*], $stdin, $stdout, $stderr, $status); }
+    #[test]
+    fn $name() {
+      integration_test($text, &[$($arg)*], $stdin, $stdout, $stderr, $status);
     }
   }
 }
 
 fn integration_test(
-  shell: &str,
   justfile: &str,
   args: &[&str],
   stdin: &str,
@@ -72,7 +65,7 @@ fn integration_test(
 
   let mut child = Command::new(&executable_path("just"))
     .current_dir(tmp.path())
-    .args(&["--shell", shell])
+    .args(&["--shell", "bash"])
     .args(args)
     .stdin(Stdio::piped())
     .stdout(Stdio::piped())