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/ietf_syslog/basic/basic.adoc

@@ -1,24 +1,15 @@
+ifdef::topdoc[:imagesdir: {topdoc}../../test/case/ietf_syslog/basic]
+
 === Syslog Basic
 ==== Description
 Add syslog actions to log to local files, then verify new log files have been created.
 
 ==== Topology
-ifdef::topdoc[]
-image::{topdoc}../../test/case/ietf_syslog/basic/topology.svg[Syslog Basic topology]
-endif::topdoc[]
-ifndef::topdoc[]
-ifdef::testgroup[]
-image::basic/topology.svg[Syslog Basic topology]
-endif::testgroup[]
-ifndef::testgroup[]
-image::topology.svg[Syslog Basic topology]
-endif::testgroup[]
-endif::topdoc[]
-==== Test sequence
+image::topology.svg[Syslog Basic topology, align=center, scaledwidth=75%]
+
+==== Sequence
 . Set up topology and attach to target DUT
 . Configure syslog on DUT to log to files '/log/bar.log' (absolute path) and 'foo' (non-absolute).
 . Verify log files /var/log/foo and /var/log/bar.log have been created
 
 
-<<<
-