tests,misc: update TESTING.md documentation

* Documentation was assuming only tests in tests/test-progs were run.
This is not true anymore since we are running linux-boot for arm
which doesn't have a directory inside test-progs and because it
is now possible to explicitly specify the bin directory
(--bin-path)

Change-Id: I4cbb02fa1c88839be45302ea64e07792e1fc81f5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24526
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>