(ci) Enable coloured mode for test-stdlib output

In normal (TTY/xterm-256color) mode, the library detects colour support
automatically. However, when running from our CI jobs, this need to be
manually enabled.

https://gitlab.perlang.org/perlang/perlang/-/merge_requests/554

Makefile

@@ -155,7 +155,8 @@ test:
 	dotnet test --configuration Release
 
 test-stdlib: stdlib
-	src/stdlib/out/tests
+# We need --colour-mode since colour support is not auto-detected in CI.
+	src/stdlib/out/tests --colour-mode ansi
 
 #
 # Steps for publishing a new release:

release-notes/v0.6.0.md

@@ -61,6 +61,7 @@
 - Run `test` job in build matrix, with and without Valgrind enabled [[!535][535]]
 - Add JUnit-based test reporting for GitLab CI [[!539][539]]
 - Enable full leak checking when running Valgrind [[!547][547]]
+- Enable coloured mode for `test-stdlib` output [[!554][554]
 
 ### Removed
 - Remove unused `Perlang.Compiler` project [[!511][511]]
@@ -139,3 +140,4 @@
 [551]: https://gitlab.perlang.org/perlang/perlang/merge_requests/551
 [552]: https://gitlab.perlang.org/perlang/perlang/merge_requests/552
 [553]: https://gitlab.perlang.org/perlang/perlang/merge_requests/553
+[554]: https://gitlab.perlang.org/perlang/perlang/merge_requests/554