feat(test): test ansi colors with stderr/stdout merged output

spec/fixtures/interleaved.cpp

@@ -0,0 +1,9 @@
+#include <iostream>
+#include <cstdio>
+
+int main() {
+    std::cout << "\033[32mstdout: \033[1mSuccess\033[0m" << std::endl;
+    std::cerr << "\033[31mstderr: \033[1mWarning\033[0m" << std::endl;
+    std::cout << "plain stdout" << std::endl;
+    return 0;
+}

spec/fixtures/syntax_error.cpp

@@ -0,0 +1,8 @@
+#include <iostream>
+
+int main() {
+    std::cout << "this will never compile" << std::endl
+    // missing semicolon above
+    undefined_function();
+    return 0
+    // missing semicolon again