Disable tests on Windows that use ed

[cakebaker]

This PR disables three tests on Windows which currently fail because there is no ed program. It's related to #39, though doesn't fix it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.82%. Comparing base (d863fe4) to head (2d9e625).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   76.79%   76.82%   +0.02%     
==========================================
  Files           9        9              
  Lines        2564     2567       +3     
  Branches      657      657              
==========================================
+ Hits         1969     1972       +3     
  Misses        466      466              
  Partials      129      129              

Flag	Coverage Δ
macos_latest	77.00% <100.00%> (+0.02%)	⬆️
ubuntu_latest	76.86% <100.00%> (+0.02%)	⬆️

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[marc-hb]

No big deal but I think it would be better not to give Windows users the wrong impression that these tests are passing on Windows. I mean like this instead:

--- a/src/ed_diff.rs
+++ b/src/ed_diff.rs
@@ -181,6 +181,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg_attr(target_os = "windows", ignore = "no 'ed' on windows")]
     fn test_permutations() {
         let target = "target/ed-diff/";
         // test all possible six-line files.

This prints the following instead of making the tests silently disappear on Windows:

test ed_diff::tests::test_permutations ... ignored, no 'ed' on windows

I can submit a PR but reverting 2d9e625 would unfortunately git conflict with #226 - so I won't submit anything yet.

PS: following #226, it would be even better to dynamically skip based on whether ed is found or not. After all, ed can be missing on Linux too! See #39 (comment). But that would require "upgrading" to a non-std test crate:

• Add ability to ignore tests at runtime. rust-lang/rust#68007