[M4] add cargoxx run/test/clean

tests/cmd_clean.cpp

@@ -0,0 +1,44 @@
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.cli;
+import cargoxx.util;
+import std;
+
+using cargoxx::cli::cmd_clean;
+
+namespace {
+
+auto fresh_dir() -> std::filesystem::path {
+    auto d = std::filesystem::temp_directory_path() /
+             std::format("cargoxx-clean-test-{}", std::random_device{}());
+    std::filesystem::create_directories(d);
+    return d;
+}
+
+} // namespace
+
+TEST_CASE("cmd_clean removes the build directory", "[cli][clean]") {
+    auto root = fresh_dir();
+    std::filesystem::create_directories(root / "build" / "debug");
+    std::ofstream{root / "build" / "debug" / "stale.txt"} << "x";
+
+    REQUIRE(cmd_clean(root).has_value());
+    REQUIRE_FALSE(std::filesystem::exists(root / "build"));
+}
+
+TEST_CASE("cmd_clean is a no-op when build/ does not exist", "[cli][clean]") {
+    auto root = fresh_dir();
+    REQUIRE(cmd_clean(root).has_value());
+}
+
+TEST_CASE("cmd_clean leaves Cargoxx.lock and flake.lock alone", "[cli][clean]") {
+    auto root = fresh_dir();
+    std::filesystem::create_directories(root / "build");
+    std::ofstream{root / "Cargoxx.lock"} << "version = 1\n";
+    std::ofstream{root / "flake.lock"} << "{}\n";
+
+    REQUIRE(cmd_clean(root).has_value());
+    REQUIRE_FALSE(std::filesystem::exists(root / "build"));
+    REQUIRE(std::filesystem::exists(root / "Cargoxx.lock"));
+    REQUIRE(std::filesystem::exists(root / "flake.lock"));
+}