tests/extmod: Skip binascii tests when hexlify/unhexlify don't exist.

These functions are only available when `MICROPY_PY_BUILTINS_BYTES_HEX` is
enabled.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/binascii_hexlify.py

@@ -1,5 +1,5 @@
 try:
-    import binascii
+    from binascii import hexlify
 except ImportError:
     print("SKIP")
     raise SystemExit
@@ -10,10 +10,10 @@ for x in (
     b"\x7f\x80\xff",
     b"1234ABCDabcd",
 ):
-    print(binascii.hexlify(x))
+    print(hexlify(x))
 
 # Two-argument version (now supported in CPython)
-print(binascii.hexlify(b"123", ":"))
+print(hexlify(b"123", ":"))
 
 # zero length buffer
-print(binascii.hexlify(b"", b":"))
+print(hexlify(b"", b":"))

tests/extmod/binascii_unhexlify.py

@@ -1,5 +1,5 @@
 try:
-    import binascii
+    from binascii import unhexlify
 except ImportError:
     print("SKIP")
     raise SystemExit
@@ -10,14 +10,14 @@ for x in (
     b"7f80ff",
     b"313233344142434461626364",
 ):
-    print(binascii.unhexlify(x))
+    print(unhexlify(x))
 
 try:
-    a = binascii.unhexlify(b"0")  # odd buffer length
+    a = unhexlify(b"0")  # odd buffer length
 except ValueError:
     print("ValueError")
 
 try:
-    a = binascii.unhexlify(b"gg")  # digit not hex
+    a = unhexlify(b"gg")  # digit not hex
 except ValueError:
     print("ValueError")