Change test that relies on xpdf not being installed

[mtorpey]

Previously, the tests in display.tst try to call Splash with "xpdf" as the viewer, and we expect an error to be thrown. This only works if xpdf is not installed, and if xpdf is installed then the tests fail.

This commit changes the test to a dummy viewer which will definitely not be installed.

[mtorpey]

From my computer on the main branch:

gap> DigraphsTestStandard();                                          
Architecture: x86_64-pc-linux-gnu-default64-kv8

testing: /home/mtorpey/.gap/pkg/Digraphs/tst/standard/attr.tst
    2705 ms (127 ms GC) and 685MB allocated for attr.tst
testing: /home/mtorpey/.gap/pkg/Digraphs/tst/standard/cliques.tst
     728 ms (284 ms GC) and 37.6MB allocated for cliques.tst
testing: /home/mtorpey/.gap/pkg/Digraphs/tst/standard/constructors.tst
     189 ms (153 ms GC) and 3.52MB allocated for constructors.tst
testing: /home/mtorpey/.gap/pkg/Digraphs/tst/standard/digraph.tst
    1115 ms (128 ms GC) and 359MB allocated for digraph.tst
testing: /home/mtorpey/.gap/pkg/Digraphs/tst/standard/display.tst
########> Diff in /home/mtorpey/.gap/pkg/Digraphs/tst/standard/display.tst:440
# Input is:
Splash("string", rec(viewer := "xpdf"));
# Expected output:
Error, the viewer "xpdf" specified in the option `viewer` is not available,
# But found:
Error, the component "type" of the 2nd argument <a record>  must be "dot" or "\
latex",
########
false

[james-d-mitchell]

Also resolves #577 !