Explain how to run tests with CMake and Bazel (#4383)

Update README.

1 files changed, 26 insertions, 2 deletions

diff --git a/README.md b/README.md
index db52b79fe..804b76379 100644
--- a/README.md
+++ b/README.md
@@ -643,8 +643,32 @@ This is experimental.
 
 ### Tests
 
-Tests are only built when googletest is found. Use `ctest` to run all the
-tests.
+Tests are only built when googletest is found.
+
+#### Running test with CMake
+
+Use `ctest -j <num threads>` to run all the tests. To run tests using all threads:
+```shell
+ctest -j$(nproc)
+```
+
+To run a single test target, use `ctest [-j <N>] -R <test regex>`. For example,
+you can run all `opt` tests with:
+```shell
+ctest -R 'spirv-tools-test_opt'
+```
+
+#### Running test with Bazel
+
+Use `bazel test :all` to run all tests. This will run tests in parallel by default.
+
+To run a single test target, specify `:my_test_target` instead of `:all`. Test target
+names get printed when you run `bazel test :all`. For example, you can run
+`opt_def_use_test` with:
+```shell
+bazel test :opt_def_use_test
+```
+
 
 ## Future Work
 <a name="future"></a>