meson: use "_" as separator in test names

":" is prettier, but meson 0.56+ doesn't like it:
src/systemd/meson.build:73: DEPRECATION: ":" is not allowed in test name "cc-sd-bus.h:c", it has been replaced with "_"
src/systemd/meson.build:73: DEPRECATION: ":" is not allowed in test name "cc-sd-bus.h:c-ansi", it has been replaced with "_"
...

Fixes #17568.

(cherry picked from commit ca121e20)