Add unit testing framework readme (#766)

test/README.md

@@ -0,0 +1,130 @@
+# MSCCL++ C++ Test Framework
+
+A lightweight, GTest-like test framework with MPI support for testing MSCCL++ C++ APIs. Defined in `framework.hpp` / `framework.cc`.
+
+## Adding a New Test (Step-by-Step)
+
+### Single-process test (unit/)
+
+1. **Create the test file** `test/unit/my_feature_tests.cc` (or `.cu` for CUDA):
+
+    ```cpp
+    #include "../framework.hpp"
+    #include <mscclpp/my_feature.hpp>
+
+    TEST(MyFeatureTest, BasicUsage) {
+      EXPECT_EQ(myFunction(), 42);
+    }
+    ```
+
+2. **Register it in CMake** — add the filename to `test/unit/CMakeLists.txt`:
+
+    ```cmake
+    target_sources(unit_tests PRIVATE
+        ...
+        my_feature_tests.cc   # <-- add here
+    )
+    ```
+
+3. **Build and run**:
+
+    ```bash
+    cmake --build build -j
+    ./build/test/unit_tests --filter=MyFeatureTest
+    ```
+
+### Multi-process test (mp_unit/)
+
+1. **Create the test file** `test/mp_unit/my_feature_tests.cc` (or `.cu`):
+
+    ```cpp
+    #include "mp_unit_tests.hpp"
+
+    TEST(MyFeatureTest, MultiRank) {
+      int rank = gEnv->rank;
+      EXPECT_GE(rank, 0);
+    }
+    ```
+
+    Use fixtures from `mp_unit_tests.hpp` (e.g., `CommunicatorTest`) if you need pre-established connections.
+
+2. **Register it in CMake** — add the filename to `test/mp_unit/CMakeLists.txt`:
+
+    ```cmake
+    target_sources(mp_unit_tests PRIVATE
+        ...
+        my_feature_tests.cc   # <-- add here
+    )
+    ```
+
+3. **Build and run**:
+
+    ```bash
+    cmake --build build -j
+    mpirun -np 2 ./build/test/mp_unit_tests --filter=MyFeatureTest
+    ```
+
+### Notes
+
+- No separate test registration step is needed — `TEST()` auto-registers via static initialization.
+- The `test_framework` static library is built from `framework.cc` in the top-level `test/CMakeLists.txt` and linked into both `unit_tests` and `mp_unit_tests`. You do not need to modify it.
+- Use `.cu` extension for files that contain CUDA kernel code; use `.cc` for host-only tests.
+- Each test binary needs a `main()` that calls `RUN_ALL_TESTS()`. See `unit/unit_tests_main.cc` (single-process) and `mp_unit/mp_unit_tests.cc` (multi-process with `Environment` setup).
+- Additional run options: `--filter=-Pattern` (exclude), `--exclude-perf-tests` (skip `PERF_TEST`s).
+
+## Macros
+
+| Macro | Behavior |
+|---|---|
+| `TEST(Suite, Name)` | Register a test. If `Suite` is a defined class, it's used as a fixture. |
+| `PERF_TEST(Suite, Name)` | Same as `TEST` but marked as perf (skippable via `--exclude-perf-tests`). |
+| `EXPECT_*` | Non-fatal assertions: `EXPECT_TRUE`, `EXPECT_FALSE`, `EXPECT_EQ`, `EXPECT_NE`, `EXPECT_LT`, `EXPECT_LE`, `EXPECT_GT`, `EXPECT_GE` |
+| `ASSERT_*` | Fatal assertions (abort test on failure): same variants as `EXPECT_*`, plus `ASSERT_NO_THROW` |
+| `FAIL()` | Fail immediately. Supports streaming: `FAIL() << "reason";` |
+| `SKIP_TEST()` | Skip the current test. Supports streaming: `SKIP_TEST() << "reason";` |
+| `CUDA_CHECK(call)` | Check a CUDA API return code, throw on error. |
+
+## Fixtures
+
+Define a class inheriting from `mscclpp::test::TestCase` with `SetUp()` / `TearDown()`, then use the class name as the suite name:
+
+```cpp
+class MyFixture : public mscclpp::test::TestCase {
+ public:
+  void SetUp() override { /* per-test setup */ }
+  void TearDown() override { /* per-test cleanup */ }
+ protected:
+  int sharedState_ = 0;
+};
+
+TEST(MyFixture, SomeTest) {
+  sharedState_ = 42;
+  EXPECT_EQ(sharedState_, 42);
+}
+```
+
+See `mp_unit/mp_unit_tests.hpp` (`BootstrapTest`, `CommunicatorTest`, etc.) for real fixture examples.
+
+## Global Environments
+
+Register an `Environment` subclass for one-time global setup/teardown (e.g., MPI bootstrap):
+
+```cpp
+class MyEnv : public mscclpp::test::Environment {
+ public:
+  void SetUp() override { /* global init */ }
+  void TearDown() override { /* global cleanup */ }
+};
+
+// In main(), before RUN_ALL_TESTS():
+mscclpp::test::TestRegistry::instance().addEnvironment(new MyEnv());
+```
+
+See `mp_unit/mp_unit_tests.cc` for the `MultiProcessTestEnv` example.
+
+## Utilities
+
+- `mscclpp::test::utils::isMainRank()` — true on MPI rank 0
+- `mscclpp::test::utils::getMPIRank()` / `getMPISize()`
+- `mscclpp::test::utils::Timer` — high-resolution timer with `start()`, `stop()`, `elapsedMilliseconds()`
+- `mscclpp::test::currentTestName()` — returns `"Suite.Name"` for the running test