Fix missing Pango support in R 3.x for openSUSE 15.6

[glin]

Fix missing Pango support in R 3.x for openSUSE 15.6. Same as #120 for Ubuntu 22 a while back. This was a new thing in SUSE 15.6, and wasn't caught by the tests because we don't test R 3.x anymore. I've added a test for R 3.6, since we have to support it longer term than usual.

The error was with running any graphics device, like png():

> png()
Error in .External2(C_X11, paste0("png::", filename), g$width, g$height,  : 
  unable to start device PNG
In addition: Warning message:
In png() : unable to open connection to X11 display ''

# This should be cairo
> getOption("bitmapType")
[1] "Xlib"

And Pango now depending on PCRE2 in SUSE 15.6:

$ zypper in pango-devel

The following 120 NEW packages are going to be installed:
  ... libpango-1_0-0 libpcre2-16-0 libpcre2-32-0 libpcre2-posix3
  ...

After the fix, the tests should pass and png() should work again