CIs set QEMU and nspawn timeouts by themselves which reflect their needs
and possibilities, so let's respect that value, instead of using one
pre-set value which might or might not work for all of them.
Both Ubuntu CI and CentOS CI set these values themselves.
We used both "qemu" and "QEMU", let's use the lower-case version everywhere
since it's also the name of the binary and the version that people are
most familiar with.
The stuff under test/ is not only for the integeration tests, but also
for various other test-related stuff, so adjust the docs a bit.
Since the TEST-64-UDEV-STORAGE fails are quite frequent now and the root
cause is yet to be discovered, let's add a kludge that attempts to retry
the test up to two more times in case it fails, so we don't
unnecessarily disturb CIs while the issue is being investigated.
Revert this commit once #21819 is sorted out.
Unfortunately, when checking the return/exit code using &&, ||, if,
while, etc., `set -e` is disabled for all nested functions as well,
which leads to incorrectly ignored errors, *sigh*.
Example:
```
set -eu
set -o pipefail
task() {
echo "task init"
echo "this should fail"
false
nonexistentcommand
echo "task end (we shouldn't be here)"
}
if ! task; then
echo >&2 "The task failed"
exit 1
else
echo "The task passed"
fi
```
```
$ bash test.sh
task init
this should fail
test.sh: line 10: nonexistentcommand: command not found
task end (we shouldn't be here)
The task passed
$ echo $?
0
```
But without the `if`, everything works "as expected":
```
set -eu
set -o pipefail
task() {
echo "task init"
echo "this should fail"
false
nonexistentcommand
echo "task end (we shouldn't be here)"
}
task
```
```
$ bash test.sh
task init
this should fail
$ echo $?
1
```
Wonderful.
