Add simple test for PR10257 (sprint(@hist_linear)).

Add simple test for PR10257 (sprint(@hist_linear)).

Test is simple, because print and sprint uses the same code for handling
@hist_* and it is not intended for playing with MAXSTRINGLEN, which is
required anyway for bigger histograms (the one generated here consists
of only 127 characters).

When PR10690 (need way to produce bigger procfs output) will be fixed,
then we should add another test for both PRs with normal histogram.