tests/integration: rm excessive run use; fix exec --preserve-fds test

[kolyshkin]

1. rm excessive run use

Bats' run should only be used when we want to check both the command
output and its non-zero exit status.

Otherwise, we can rely on implicit exit code check (as the tests are
run with set -e), or use if, etc.

2. fix "runc exec --preserve-fds" test case

1. Fix the check, mostly by changing cat hello to echo hello,
   and checking for "hello" (whole string) rather than *"hello"* (substring).

   Previously, cat hello generated cat: hello: no such file or directory
   error message, which run added to $output and
   so the check for $output containing hello worked!

2. Simplify the test by not using the subshell and the run.
   The only catch is, fd 3 is used by bats itself, so we have to use
   fd 4 and thus --preserve-fds 2.

3. simplify teardown_running_container

In its current form, it is complicated, unreliable, and error prone (TOCTOU etc)

Using runc delete -f will kill and remove any container, running or not,
and it won't error if a container with a given name does not exist.

[giuseppe]

LGTM. Not a maintainer but I've introduced the issue fixed by the second commit

[kolyshkin]

Rebased on top of #2666 to fix CI

[kolyshkin]

=== RUN   TestExecInTTY
2820    execin_test.go:349: unexpected carriage-return in output
2821--- FAIL: TestExecInTTY (0.10s)

Hmm I think I've seen it before...

Update: this is another time we see #2425

[kolyshkin]

restarted CI

[kolyshkin]

@AkihiroSuda @mrunalp PTAL

[kolyshkin]

@AkihiroSuda PTAL