modstruct: Improve compliance with python3

While checking whether we can enable -Wimplicit-fallthrough, I encountered
a diagnostic in mp_binary_set_val_array_from_int which led to discovering
the following bug:
```
>>> struct.pack("xb", 3)
b'\x03\x03'
```
That is, the next value (3) was used as the value of a padding byte, while
standard Python always fills "x" bytes with zeros.  I initially thought
this had to do with the unintentional fallthrough, but it doesn't.
Instead, this code would relate to an array.array with a typecode of
padding ('x'), which is ALSO not desktop Python compliant:
```
>>> array.array('x', (1, 2, 3))
array('x', [1, 0, 0])
```
Possibly this is dead code that used to be shared between struct-setting
and array-setting, but it no longer is.

I also discovered that the argument list length for struct.pack
and struct.pack_into were not checked, and that the length of binary data
passed to array.array was not checked to be a multiple of the element
size.

I have corrected all of these to conform more closely to standard Python
and revised some tests where necessary.  Some tests for micropython-specific
behavior that does not conform to standard Python and is not present
in CircuitPython was deleted outright.

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/basics/struct1.py b/tests/basics/struct1.py
index db34342a1..107006cc3 100644
--- a/tests/basics/struct1.py
+++ b/tests/basics/struct1.py
@@ -39,6 +39,28 @@ print(v == (10, 100, 200, 300))
 # network byte order
 print(struct.pack('!i', 123))
 
+# too short / too long arguments
+buf = bytearray(b'>>>123<<<')
+try:
+    struct.pack_into('bb', buf, 0, 3)
+except:
+    print('struct.error')
+
+try:
+    struct.pack_into('bb', buf, 0, 3, 1, 4)
+except:
+    print('struct.error')
+
+try:
+    struct.pack('bb', 3)
+except:
+    print('struct.error')
+
+try:
+    struct.pack('bb', 3, 1, 4)
+except:
+    print('struct.error')
+
 # check that we get an error if the buffer is too small
 try:
     struct.unpack('I', b'\x00\x00\x00')
@@ -96,3 +118,6 @@ try:
     print(struct.unpack_from('<b', buf, -11))
 except:
     print('struct.error')
+
+# check padding bytes
+print(struct.pack("xb", 3))