[DOC] Specify ways to run bootstrap tests

doc/contributing/testing_ruby.md

@@ -20,10 +20,11 @@ We can run any of the make scripts [in parallel](building_ruby.md#label-Running+
     make btest OPTS=-v
     ```
 
-    To run an individual bootstrap test, we can set the filename in the environment variable `BTESTS`:
+    To run individual bootstrap tests, we can either specify a list of filenames or use the `--sets` flag in the variable `BTESTS`:
 
     ```
-    make btest BTESTS=bootstraptest/test_gc.rb
+    make btest BTESTS="bootstraptest/test_fork.rb bootstraptest/tes_gc.rb"
+    make btest BTESTS="--sets=fork,gc"
     ```
 
     If we want to run the bootstrap test suite on Ruby (not Miniruby), we can use: