[#13] More libm integration fallout

This commit fixes two issues:

1. "make test" was not rerunnable because the output file created in the libm test had weird permissions of u+r, g+rw, o+rw. This meant that the "open file:newversion" call in the libm test failed. The cause of this issue is that we change the umask in a previous test but never restore it. This is now fixed.
2. If the M and UTF-8 tests (ydbposixtest_m and ydbposixtest_utf8 as set-up in CMakeLists.txt) are run concurrently (e.g. with ctest -j N), the libm.out file may disappear while being examined as both processes are writing to the same file. This showed up in this pipeline failure: https://gitlab.com/YottaDB/DBMS/YDBOcto/-/jobs/5759358069. This is fixed by writing to two separate files by $ZCHSET.