TST: Use text placeholders for empty legends

These tests use `remove_text=True` and set legend labels to empty
strings. However, they are still affected by font metrics because even
the empty string is as tall as the line height (which is calculated from
the height of the "lp" string.)

Author: QuLogic

lib/matplotlib/tests/baseline_images/test_axes/stem.png

@@ -4593,7 +4593,7 @@ def test_hist_stacked_weighted():
 
 
 @image_comparison(['stem.png'], style='mpl20', remove_text=True)
-def test_stem():
+def test_stem(text_placeholders):
     x = np.linspace(0.1, 2 * np.pi, 100)
 
     fig, ax = plt.subplots()

lib/matplotlib/tests/baseline_images/test_backend_pdf/hatching_legend.pdf

@@ -296,8 +296,8 @@ def test_pdfpages_fspath():
         pdf.savefig(plt.figure())
 
 
-@image_comparison(['hatching_legend.pdf'])
-def test_hatching_legend():
+@image_comparison(['hatching_legend.pdf'], style='mpl20')
+def test_hatching_legend(text_placeholders):
     """Test for correct hatching on patches in legend"""
     fig = plt.figure(figsize=(1, 2))
 

lib/matplotlib/tests/baseline_images/test_bbox_tight/bbox_inches_tight.pdf

@@ -11,9 +11,9 @@
 from mpl_toolkits.axes_grid1.inset_locator import inset_axes
 
 
-@image_comparison(['bbox_inches_tight'], remove_text=True,
+@image_comparison(['bbox_inches_tight'], remove_text=True, style='mpl20',
                   savefig_kwarg={'bbox_inches': 'tight'})
-def test_bbox_inches_tight():
+def test_bbox_inches_tight(text_placeholders):
     #: Test that a figure saved using bbox_inches='tight' is clipped correctly
     data = [[66386, 174296, 75131, 577908, 32015],
             [58230, 381139, 78045, 99308, 160454],