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/infix_containers/container_basic/container_basic.adoc

@@ -1,3 +1,5 @@
+ifdef::topdoc[:imagesdir: {topdoc}../../test/case/infix_containers/container_basic]
+
 === Container basic
 ==== Description
 Verify that a simple web server container can be configured to run
@@ -7,18 +9,9 @@ simple GET request for index.html and checking for a key phrase.
 The RPC actions: stop + start, and restart are also verified.
 
 ==== Topology
-ifdef::topdoc[]
-image::{topdoc}../../test/case/infix_containers/container_basic/topology.svg[Container basic topology]
-endif::topdoc[]
-ifndef::topdoc[]
-ifdef::testgroup[]
-image::container_basic/topology.svg[Container basic topology]
-endif::testgroup[]
-ifndef::testgroup[]
-image::topology.svg[Container basic topology]
-endif::testgroup[]
-endif::topdoc[]
-==== Test sequence
+image::topology.svg[Container basic topology, align=center, scaledwidth=75%]
+
+==== Sequence
 . Set up topology and attach to target DUT
 . Set hostname to 'container-host'
 . Create container 'web' from bundled OCI image
@@ -30,5 +23,3 @@ endif::topdoc[]
 . Verify container 'web' is reachable on http://container-host.local:91
 
 
-<<<
-