Friends, thank you so much for your kind help! Indeed: it turned out
that if /dev/shm is mounted with noexec then "--root=/dev/shm" is
failing. If you would like to ensure that the git tests are succeeding
regardless of what are the mount flags on some system or what some
distro's packagers are doing (more portability), maybe it is possible
to modify the git test system - to make it so that "askpass" always
gets launched from a native original directory, instead of being
copied to some "root" place where it could fail?

By the way I have completed a git packaging build: just these 12
mentioned tests are failing, everything else is fine. So for me atm
the possible workarounds are either GIT_SKIP_TESTS='t0300 t0301 t0302
t5003 t5411 t5540 t5541 t5550 t5551 t5559 t5563 t5564', or "mkdir -p
/tmp/git-test" then GIT_TEST_OPTS="--root=/tmp/git-test --verbose-log"
instead of a /dev/shm/git-test