tests: Add testcase for str.center().

tests/basics/string_center.py

+try:
+    str.center
+except:
+    import sys
+    print("SKIP")
+    sys.exit()
+
+print("foo".center(0))
+print("foo".center(1))
+print("foo".center(3))
+print("foo".center(4))
+print("foo".center(5))
+print("foo".center(6))
+print("foo".center(20))