Fix locale dependent behaviour in run-test.sh

'stat ... | grep Modify' fails on non english locales. stat -c '%y'
used in most of the places, but one exception remained.
10 jobs for test-fix in 8 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #6041161
gst-macos-10.15
meson macos

00:04:35

passed #6041158
meson-mingw-w64-build

00:06:10

manual #6041162
docker windows 1809 allowed to fail manual
meson msys2
passed #6041156
meson-shared-build

00:06:39

passed #6041157
meson-static-build

00:06:30

passed #6041159
docker windows 1809
meson vs2017 amd64

00:01:21

passed #6041160
docker windows 1809
meson vs2017 x86

00:01:20

passed #6041155
mingw-build

00:07:00

passed #6041153
shared-build

00:08:10

passed #6041154
static-build

00:06:08