tests: Add testcase for open(..., "a").

tests/io/open_append.py

+import sys
+try:
+    import _os as os
+except ImportError:
+    import os
+
+if not hasattr(os, "unlink"):
+    print("SKIP")
+    sys.exit()
+
+try:
+    os.unlink("testfile")
+except OSError:
+    pass
+
+# Should create a file
+f = open("testfile", "a")
+f.write("foo")
+f.close()
+
+f = open("testfile")
+print(f.read())
+f.close()
+
+f = open("testfile", "a")
+f.write("bar")
+f.close()
+
+f = open("testfile")
+print(f.read())
+f.close()