added playground project and made (again) the common.cmake behave when included multiple times
diff --git a/scripts/playground/test_output/playground.txt b/scripts/playground/test_output/playground.txt
new file mode 100644
index 0000000..0266018
--- /dev/null
+++ b/scripts/playground/test_output/playground.txt
@@ -0,0 +1,13 @@
+[doctest] run with "--help" for options
+== TEST CASE ==================================================================
+test.cpp(0)
+dev stuff
+
+test.cpp(0) ERROR!
+  CHECK( 4 == 5 )
+with expansion:
+  CHECK( 4 == 5 )
+
+===============================================================================
+[doctest] test cases:    1 |    0 passed |    1 failed |    0 skipped
+[doctest] assertions:    1 |    0 passed |    1 failed |