tests: Add test for str.splitlines().

tests/basics/string_splitlines.py

+try:
+    str.splitlines
+except:
+    import sys
+    print("SKIP")
+    sys.exit()
+
+print("foo\nbar".splitlines())
+print("foo\nbar\n".splitlines())
+print("foo\nbar".splitlines(True))
+print("foo\nbar\n".splitlines(True))
+print("foo\nbar".splitlines(keepends=True))
+print("foo\nbar\n".splitlines(keepends=True))