tests/run-tests: Use -BS flags when running CPython.

The use of -S ensures that only the CPython standard library is accessible,
which makes tests run the same regardless of any site-packages that are
installed.  It also improves start-up time of CPython, reducing the overall
time spent running the test suite.

tests/basics/containment.py is updated to work around issue with old Python
versions not being able to str-format a dict-keys object, which becomes
apparent when -S is used.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/containment.py

@@ -1,8 +1,8 @@
 # sets, see set_containment
 for i in 1, 2:
     for o in {1:2}, {1:2}.keys():
-        print("{} in {}: {}".format(i, o, i in o))
-        print("{} not in {}: {}".format(i, o, i not in o))
+        print("{} in {}: {}".format(i, str(o), i in o))
+        print("{} not in {}: {}".format(i, str(o), i not in o))
 
 haystack = "supercalifragilistc"
 for needle in [haystack[i:] for i in range(len(haystack))]: