Run subset tests with C-locale One problematic input is `--instance=wdth=112.5` which is parsed using the users local (`strtof`). So while in C or US locales it yields 112.5 in some European locales it yields 112 as the dot is not valid in this position. A European user might pass "112,5" instead but for the test the C locale is assumed, so enforce it. Fixes #4854