disable tests that fail on default windows installs (#890)

cat is not available as a command on a default windows install.
This disables the command test that uses cat during tests.  It also
disables `test_cmd_fail` even though it passes, because the test
is testing the wrong thing in this case.

tauri-api/src/command.rs

@@ -93,9 +93,9 @@ pub fn binary_command(binary_name: String) -> crate::Result<String> {
 #[cfg(test)]
 mod test {
   use super::*;
-  use crate::Error;
   use std::io;
 
+  #[cfg(not(windows))]
   #[test]
   // test the get_output function with a unix cat command.
   fn test_cmd_output() {
@@ -115,9 +115,12 @@ mod test {
     }
   }
 
+  #[cfg(not(windows))]
   #[test]
   // test the failure case for get_output
   fn test_cmd_fail() {
+    use crate::Error;
+
     // queue up a string with cat in it.
     let cmd = String::from("cat");