src/tests: Delete graves in substitution in ibus-desktop-testing-runner
src/tests: Delete graves in substitution in ibus-desktop-testing-runner

Delete the single quotations to enclose grave chracters because
DASH saves the single quoted '`id -u`' as the raw string in the command
substitution not to be extracted.

BUG=https://github.com/ibus/ibus/issues/2189

fujiwarat/ibus,ibus/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,fujiwarat/ibus,ibus/ibus