Attach to test in container from debugglibc.sh

Attach to test in container from debugglibc.sh

Some test cases are meant to be ran inside the container infrastructure
and make check automatically runs them as such.  However, running a
single test case in a container without make check is useful.

This patch adds a new --tool option to testrun.sh that makes this easy,
as well as it adds a new option (-c or --in-container) to debugglibc.sh,
which causes the program under test to be ran in a container (with
WAIT_FOR_DEBUGGER=1), then automatically attaches GDB to it.

Automatically detecting if a test case is supposed to be ran inside a
container is harder (if not impossible), as Carlos pointed out [1],
however, this patch makes it easier to do it manually:

  Using testrun.sh with containerized test:

    $ ./testrun.sh --tool=container /absolute/path/to/program

  Using debugglibc.sh with containerized test:

    $ ./debugglibc.sh -c /absolute/path/to/program

Note: running these commands with relative paths causes error and
warning messages to be displayed, although the test case might succeed.

For example, with relative path:

  $ ./testrun.sh --tool=container elf/tst-ldconfig-bad-aux-cache
  error: subprocess failed: execv
  error:   unexpected error output from subprocess
  /sbin/ldconfig: Warning: ignoring configuration file that cannot be opened: /etc/ld.so.conf: No such file or directory
  info: f    0    1064   /var/cache/ldconfig/aux-cache            20 aux-cache
  [...]

Whereas with absolute paths, the errors and warnings are gone:

  $ ./testrun.sh --tool=container $PWD/elf/tst-ldconfig-bad-aux-cache
  info: f    0    1064   /var/cache/ldconfig/aux-cache            20 aux-cache
  [...]

[1] https://sourceware.org/ml/libc-alpha/2019-11/msg00873.html

Reviewed-by: Carlos O'Donell <carlos@redhat.com>