tests: check /proc availability, and go-around if it is incomplete

tests: check /proc availability, and go-around if it is incomplete

Unavailability of /proc is fatal for kill, and continuing with the test
in that case does not make sense as it will only mean false positive
errors.

Where /proc/<pid>/status file(s) does not exist the check will perform
opportunistic sleep with assumption the test_sigreceive will be ready to
be killed if it has some time to init.

[kzak@redhat.com: - remove dependence on gawk, just use shell
                  - fix typo in "test /proc"]

CC: Ruediger Meier <sweet_f_a@gmx.de>
Reference: https://travis-ci.org/rudimeier/util-linux/jobs/24561058
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>