Add Windows-specific comments to testing guide

Fixes #27936

Author: steveklabnik

src/doc/trpl/testing.md

@@ -120,13 +120,26 @@ And that's reflected in the summary line:
 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
 ```
 
-We also get a non-zero status code:
+We also get a non-zero status code. We can use `$?` on OS X and Linux:
 
 ```bash
 $ echo $?
 101
 ```
 
+On Windows, if you’re using `cmd`:
+
+```bash
+> echo %ERRORLEVEL%
+```
+
+And if you’re using PowerShell:
+
+```bash
+> echo $LASTEXITCODE # the code itself
+> echo $? # a boolean, fail or succeed
+```
+
 This is useful if you want to integrate `cargo test` into other tooling.
 
 We can invert our test's failure with another attribute: `should_panic`: