proc: test /proc/self symlink

There are plans to change how /proc/self result is calculated,
for that a test is necessary.

Use direct system call because of this whole getpid caching story.

Link: http://lkml.kernel.org/r/20180627195103.GB18113@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/proc/.gitignore

@@ -9,3 +9,4 @@
 /proc-uptime-001
 /proc-uptime-002
 /read
+/self