Include `max_elements_per_file` in `save_fixture` call in `PlaintextFixture`

[griff-rees]

PlaintextFixture has a max_plaintext_per_fixture_file attribute designed to be passed to save_fixture. It currently isn't passed.

This may explain why a SIGKILL error can occur. An example from importing bl_lwm:

[15:57:20] INFO     Extracting: 'bl_lwm-plaintext/0003537_plaintext.zip' ...                             plaintext.py:483
[15:58:25] INFO     Extracting: 'bl_lwm-plaintext/0003538_plaintext.zip' ...                             plaintext.py:483
[15:59:29] INFO     Extracting: 'bl_lwm-plaintext/0003539_plaintext.zip' ...                             plaintext.py:483
[16:00:32] INFO     Extracting: 'bl_lwm-plaintext/0003540_plaintext.zip' ...                             plaintext.py:483
[16:01:35] INFO     Extracting: 'bl_lwm-plaintext/0003541_plaintext.zip' ...                             plaintext.py:483
[16:02:40] INFO     Extracting: 'bl_lwm-plaintext/0003548_plaintext.zip' ...                             plaintext.py:483 
100% ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━107/107  
Compressed configs  :   6% ━━╺━━━━━━━━━━━━━━━━━━375,511/6,424,616  [ 0:01:32 < 0:25:42 , 3,923 it/s ]

fish:Job 1, 'poetry run a2t2f-plaintext bl_l…' terminated by signal SIGKILL (Forced quit)