SCons: Build thirdparty code in own env, disable warnings

Also remove unnecessary `Export('env')` in other SCsubs,
Export should only be used when exporting *new* objects.

main/tests/SCsub

@@ -5,9 +5,5 @@ Import('env')
 env.tests_sources = []
 env.add_source_files(env.tests_sources, "*.cpp")
 
-Export('env')
-
-# SConscript('math/SCsub');
-
 lib = env.add_library("tests", env.tests_sources)
 env.Prepend(LIBS=[lib])