tests/extmod: Add btree test for errors raised by btree DB library.

This test now passes given the previous two commits.

tests/extmod/btree_error.py

@@ -0,0 +1,42 @@
+# Test that errno's propagate correctly through btree module.
+
+try:
+    import btree, uio, uerrno
+
+    uio.IOBase
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+
+class Device(uio.IOBase):
+    def __init__(self, read_ret=0, ioctl_ret=0):
+        self.read_ret = read_ret
+        self.ioctl_ret = ioctl_ret
+
+    def readinto(self, buf):
+        print("read", len(buf))
+        return self.read_ret
+
+    def ioctl(self, cmd, arg):
+        print("ioctl", cmd)
+        return self.ioctl_ret
+
+
+# Invalid pagesize; errno comes from btree library
+try:
+    db = btree.open(Device(), pagesize=511)
+except OSError as er:
+    print("OSError", er.args[0] == uerrno.EINVAL)
+
+# Valid pagesize, device returns error on read; errno comes from Device.readinto
+try:
+    db = btree.open(Device(-1000), pagesize=512)
+except OSError as er:
+    print(repr(er))
+
+# Valid pagesize, device returns error on seek; errno comes from Device.ioctl
+try:
+    db = btree.open(Device(0, -1001), pagesize=512)
+except OSError as er:
+    print(repr(er))

tests/extmod/btree_error.py.exp

@@ -0,0 +1,6 @@
+OSError True
+read 24
+OSError(1000,)
+read 24
+ioctl 2
+OSError(1001,)