proc: test /proc/thread-self symlink

Same story: I have WIP patch to make it faster, so better have a test
as well.

Link: http://lkml.kernel.org/r/20180627195209.GC18113@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/proc.h

@@ -14,6 +14,11 @@ static inline pid_t sys_getpid(void)
 	return syscall(SYS_getpid);
 }
 
+static inline pid_t sys_gettid(void)
+{
+	return syscall(SYS_gettid);
+}
+
 static inline bool streq(const char *s1, const char *s2)
 {
 	return strcmp(s1, s2) == 0;