tests/unix/extra_coverage: Don't test stream objs with NULL write fun.

This behaviour of a NULL write C method on a stream that uses the write
adaptor objects is no longer supported.  It was only ever used by the
coverage build for testing the fail path of mp_get_stream_raise().

ports/unix/coverage.c

@@ -118,7 +118,6 @@ STATIC mp_uint_t stest_read2(mp_obj_t o_in, void *buf, mp_uint_t size, int *errc
 
 STATIC const mp_rom_map_elem_t rawfile_locals_dict_table2[] = {
     { MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mp_stream_read_obj) },
-    { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
 };
 
 STATIC MP_DEFINE_CONST_DICT(rawfile_locals_dict2, rawfile_locals_dict_table2);

tests/unix/extra_coverage.py

@@ -38,11 +38,7 @@ except OSError:
 stream.set_error(0)
 print(stream.ioctl(0, bytearray(10))) # successful ioctl call
 
-stream2 = data[3] # is textio and sets .write = NULL
-try:
-    print(stream2.write(b'1')) # attempt to call NULL implementation
-except OSError:
-    print('OSError')
+stream2 = data[3] # is textio
 print(stream2.read(1)) # read 1 byte encounters non-blocking error with textio stream
 
 # test BufferedWriter with stream errors

tests/unix/extra_coverage.py.exp

@@ -90,7 +90,6 @@ b'123'
 b'123'
 OSError
 0
-OSError
 None
 None
 frzstr1