[MXNET-23] add README for test directory (apache#11390)

* add README for tests

* update read me with relative path to test file

* update test readme to include instructions for running with docker

* retrigger

* another retrigger

tests/README.md

@@ -0,0 +1,58 @@
+# Testing MXNET
+
+## Running CPP Tests
+
+1. Install [cmake](https://cmake.org/install/)
+1. Create a build directory in the root of the mxnet project
+    ```
+    mkdir build
+    cd build
+    ```
+1. Generate your Makefile and build along with the tests with cmake (specify appropraite flags)
+    ```
+    cmake -DUSE_CUDNN=ON -DUSE_CUDA=ON -DUSE_MKLDNN=ON -DBLAS=Open -DCMAKE_BUILD_TYPE=Debug .. && make
+    ```
+1.  Run tests
+    ```
+    ctest --verbose
+    ```
+
+1. The following will run all the tests the in `cpp` directory. To run just your test file replace the following in your `tests/CMakeLists.txt`
+    ```
+    file(GLOB_RECURSE UNIT_TEST_SOURCE "cpp/*.cc" "cpp/*.h")
+    ```
+    with
+    ```
+    file(GLOB_RECURSE UNIT_TEST_SOURCE "cpp/test_main.cc" "cpp/{RELATIVE_PATH_TO_TEST_FILE}")
+    ```
+
+### Building with Ninja
+
+Ninja is a build tool (like make) that prioritizes building speed. If you will be building frequently, we recommend you use ninja
+
+1. Download Ninja via package manager or directly from [source](https://github.com/ninja-build/ninja)
+    ```
+    apt-get install ninja-build
+    ```
+1. When running cmake, add the `-GNinja` flag to specify cmake to generate a Ninja build file
+    ```
+    cmake -DUSE_CUDNN=ON -DUSE_CUDA=ON -DUSE_MKLDNN=ON -DBLAS=Open -GNinja -DCMAKE_BUILD_TYPE=Debug ..
+    ```
+1. Run the ninja build file with
+    ```
+    ninja
+    ```
+    
+## Runing Python Tests Within Docker
+
+1. To run tests inside docker run the following comamdn
+    ```
+    ci/build.py --platform {PLATFORM} /work/runtime_functions.sh {RUNTIME_FUNCTION}
+    ```
+An example for running python tests would be
+```
+ci/build.py --platform build_ubuntu_cpu_mkldnn /work/runtime_functions.sh unittest_ubuntu_python3_cpu PYTHONPATH=./python/ nosetests-2.7 tests/python/unittest
+```
+
+
+