test: Update 'make test' to run more tests

The standard sandbox board cannot run the of-platdata test since it needs
SPL. Also, we should test the flat tree version of sandbox.

Add these tests to the default test script.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/run

 #!/bin/sh
 
-# Run all tests
+# Run all tests that the standard sandbox build can support
 ./test/py/test.py --bd sandbox --build
+
+# Run tests which require sandbox_spl
+./test/py/test.py --bd sandbox_spl --build -k test/py/tests/test_ofplatdata.py
+
+# Run tests for the flat DT version of sandbox
+./test/py/test.py --bd sandbox_flattree --build