tests/open_plus: Add tests for "r+", "w+" open modes.

tests/io/open_plus.py

@@ -0,0 +1,42 @@
+import sys
+try:
+    import uos as os
+except ImportError:
+    import os
+
+if not hasattr(os, "unlink"):
+    print("SKIP")
+    sys.exit()
+
+# cleanup in case testfile exists
+try:
+    os.unlink("testfile")
+except OSError:
+    pass
+
+try:
+    f = open("testfile", "r+b")
+    print("Unexpectedly opened non-existing file")
+except OSError:
+    print("Expected OSError")
+    pass
+
+f = open("testfile", "w+b")
+f.write(b"1234567890")
+f.seek(0)
+print(f.read())
+f.close()
+
+# Open with truncation
+f = open("testfile", "w+b")
+f.write(b"abcdefg")
+f.seek(0)
+print(f.read())
+f.close()
+
+# Open without truncation
+f = open("testfile", "r+b")
+f.write(b"1234")
+f.seek(0)
+print(f.read())
+f.close()