NO-JIRA: chore(tests/containers/workbenches/rstudio): trailing slash

Let's not use trailing slash for the directory. When this variable is
used and some value should be append to it, it should be done always as:
`APP_ROOT_HOME/some-path`. So the forward slash is added there and we
avoid having double slash in our outputs that may be misleading (e.g.
empty variable value in the path).

tests/containers/workbenches/rstudio/rstudio_test.py

@@ -22,7 +22,7 @@
 class TestRStudioImage:
     """Tests for RStudio Workbench images in this repository."""
 
-    APP_ROOT_HOME = "/opt/app-root/src/"
+    APP_ROOT_HOME = "/opt/app-root/src"
 
     @allure.issue("RHOAIENG-17256")
     def test_rmd_to_pdf_rendering(self, image: str) -> None: