unittest: make 1652 stable across collations

The previous coding used a format string whose output depended on the
current locale of the environment running the test. Since the gist of
the test is to have a format string, with the actual formatting being
less important, switch to a more stable formatstring with decimals.

Reported-by: Marcel Raad
Closes #3234
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>