test: testing tarantool in background mode

Before a commit ec1af129 ("box: do not close xlog file descriptors in
the atfork handler") there was a bug when Tarantool with enabled
background mode via environment variable could lead a crash:

NO_WRAP
```
$ TT_PID_FILE=tarantool.pid TT_LOG=tarantool.log TT_BACKGROUND=true TT_LISTEN=3301 tarantool -e 'box.cfg{}'
$ tail -3 tarantool.log
2021-11-02 16:05:43.672 [2341202] main init.c:696 E> LuajitError: cannot read stdin: Resource temporarily unavailable
2021-11-02 16:05:43.672 [2341202] main F> fatal error, exiting the event loop
2021-11-02 16:05:43.672 [2341202] main F> fatal error, exiting the event loop
```
NO_WRAP

With commit ec1af129 ("box: do not close xlog file descriptors in
the atfork handler") described bug could not be reproduced.

Proposed patch adds a test that starts Tarantool in background mode
enabled via box.cfg option and via environment variable TT_BACKGROUND to
make sure this behaviour will not be broken in a future.

Closes #6128

NO_DOC=test