test/case: simplify AsciiDoc image references

This commit greatly simplifies AsciiDoc image references in generated
Readme.adoc files.  The two focused use-cases that remain after this
change are working references in:

 - Generated output/images/test-report.pdf
 - Viewing test's Readme.adoc from GitHub

Previously we aimed to have working images also when the test's Readme
was included in the parent directory's Readme.adoc.  This, however, is
not supported as of this commit.  It seems unlikely also to ever be a
supported feature of AsciiDoc on GitHub, for details, see the following
issue: <https://github.com/github/markup/issues/1095>

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/case/misc/operational_all/operational_all.adoc

@@ -1,24 +1,15 @@
+ifdef::topdoc[:imagesdir: {topdoc}../../test/case/misc/operational_all]
+
 === Get operational
 ==== Description
 Basic test just to get operational from test-config without errors.
 
 ==== Topology
-ifdef::topdoc[]
-image::{topdoc}../../test/case/misc/operational_all/topology.svg[Get operational topology]
-endif::topdoc[]
-ifndef::topdoc[]
-ifdef::testgroup[]
-image::operational_all/topology.svg[Get operational topology]
-endif::testgroup[]
-ifndef::testgroup[]
-image::topology.svg[Get operational topology]
-endif::testgroup[]
-endif::topdoc[]
-==== Test sequence
+image::topology.svg[Get operational topology, align=center, scaledwidth=75%]
+
+==== Sequence
 . Set up topology and attach to target DUT
 . Copy test-config to running configuration
 . Get all Operational data from 'target', verify there are no errors
 
 
-<<<
-