Add more information to traffic suite timeout failures

In particular, have the resulting badmatch contain the starting and
ending time. There are still sporadic failures on slow hosts.
Lighten up on suite timetraps

Due to sporadic timeouts one some (slow) hosts.
Add gen_tcp suite

This is initially to identify the source of some flakiness on Windows.
Minor traffic suite fix

Point was to test that Session-Id was not undefined. Instead, test case
send_error just returned false.
Add examples testcase to help identify timetrap failures

There are still enslave/1 failures on some hosts despite a 2 minute
timetrap.
Teach c:ls/1 to show non-directory files

In an email to erlang-questions, Bengt Kleberg wrote:

    When I use c:ls/1 it reminds me so much of Unix "ls" that I
    expect c:ls("filename") to work. The resulting error surprises
    me every time (not the same day).

While teaching c:ls/1 to show non-directory files, update the
error handling to make use of the POSIX error codes from
file:list_dir/1 and file:format_error/1 (which had not been
invented when the c module was first implemented).

Suggested-by: Bengt Kleberg
Test-suite-by: Bengt Kleberg
Merge branch 'anders/diameter/timetraps/OTP-10914' into maint

* anders/diameter/timetraps/OTP-10914:
  Add examples testcase to help identify timetrap failures
  Minor traffic suite fix
  Add gen_tcp suite
  Lighten up on suite timetraps
  Add more information to traffic suite timeout failures
Merge branch 'bjorn/stdlib/improve-ls/OTP-11108' into maint

* bjorn/stdlib/improve-ls/OTP-11108:
  Teach c:ls/1 to show non-directory files