fixup 558cba13: bump tests repo to published version

558cba13 broke submodule update --init:

    git clone https://git.scc.kit.edu/IPDSnelting/mjtest.git && \
          cd mjtest && \
          git submodule update --init

    Cloning into 'mjtest'...
    remote: Enumerating objects: 788, done.
    remote: Counting objects: 100% (788/788), done.
    remote: Compressing objects: 100% (391/391), done.
    remote: Total 788 (delta 437), reused 725 (delta 392)
    Receiving objects: 100% (788/788), 162.16 KiB | 3.45 MiB/s, done.
    Resolving deltas: 100% (437/437), done.
    Submodule 'tests' (https://git.scc.kit.edu/IPDSnelting/mjtest-tests) registered for path 'tests'
    Cloning into '/tmp/mjtest/tests'...
    warning: redirecting to https://git.scc.kit.edu/IPDSnelting/mjtest-tests.git/
    warning: redirecting to https://git.scc.kit.edu/IPDSnelting/mjtest-tests.git/
    warning: redirecting to https://git.scc.kit.edu/IPDSnelting/mjtest-tests.git/
    error: Server does not allow request for unadvertised object 78b27a109cb1a1a6bc6d3a6e0f57b5018305530c
    Fetched in submodule path 'tests', but it did not contain 78b27a109cb1a1a6bc6d3a6e0f57b5018305530c. Direct fetching of that commit failed.

tests @ e1979f10

-Subproject commit 78b27a109cb1a1a6bc6d3a6e0f57b5018305530c
+Subproject commit e1979f1061c6e13c5d8bd407145bbb111c9101b7