planemo test error for output file name

My <test> block includes:

      <output name="pdf_file" file="hist_box.pdf" />

I ran planemo t my_galaxy.xml and get error saying
Test file (hist_box.pdf) is missing. If I comment this line, planemo t is successful.

By the way, I even ran planemo t ggplot2_pca.xml ( and get the error:
Test file (ggplot_pca_result1.pdf) is missing.

What is the problem? Many thanks in advance.