Add bitmap loading test

tests/circuitpython/_loadbmp16.py

@@ -0,0 +1,31 @@
+import displayio
+import bitmaptools
+
+
+def loadbmp16(filename, width=320, height=240):
+    """This specialized routine loads 16bpp uncompressed bmp files with a
+    70-byte header. It is not appropriate for generic bmp files."""
+
+    bitmap = displayio.Bitmap(width, height, 65536)
+    with open(filename, "rb") as f:
+        f.seek(70)
+        bitmaptools.readinto(
+            bitmap,
+            f,
+            bits_per_pixel=16,
+            element_size=2,
+            swap_bytes_in_element=True,
+            reverse_rows=True,
+        )
+
+    return bitmap
+
+
+if __name__ == "__main__":
+    if "/" in __file__:
+        here = __file__.rsplit("/", 1)[0]
+    else:
+        here = "."
+    b = loadbmp16(here + "/minerva16.bmp")
+    print(b[0, 0])
+    print(b[160, 160])