scons: Add a "check" target to run all unit tests.

Except:
- u_cache_test -- too long
- translate_test -- unreliable (it's probably testing corner cases that
  translate module doesn't care about.)

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

SConstruct

@@ -84,6 +84,11 @@ env.Append(CPPPATH = [
 #print env.Dump()
 
 
+# Add a check target for running tests
+check = env.Alias('check')
+env.AlwaysBuild(check)
+
+
 #######################################################################
 # Invoke host SConscripts 
 #