[test] upgrade unity

[r00t-]

fix #98

we're using a version of unity is EIGHT YEARS old, never updated since 2013,
this pulls in the last release from 2021.

[r00t-]

builds on that weird distro:

$ docker run -it --rm alpine /bin/ash -c 'apk add --no-cache build-base git util-linux-dev && git clone -b upgrade_unity_to_v2.5.2 https://github.com/r00t-/libsml.git && cd libsml && make'
[...]
88 Tests 0 Failures 0 Ignored 
OK
make[1]: Leaving directory '/libsml/test'

[r00t-]

can anybody check if this causes a regression on #41 or #19?
@andig @hmueller01

[m-reuter]

I could verify that this also works on Raspbian GNU/Linux 10 (buster)