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_dhcp/server_basic/server_basic.adoc

@@ -1,3 +1,5 @@
+ifdef::topdoc[:imagesdir: {topdoc}../../test/case/infix_dhcp/server_basic]
+
 === DHCP Server Basic
 ==== Description
 Verify basic DHCP server functionality.  The server is set up to only
@@ -7,18 +9,9 @@ other options are sent by checking, e.g., that the client has not set up
 a default route to the server.
 
 ==== Topology
-ifdef::topdoc[]
-image::{topdoc}../../test/case/infix_dhcp/server_basic/topology.svg[DHCP Server Basic topology]
-endif::topdoc[]
-ifndef::topdoc[]
-ifdef::testgroup[]
-image::server_basic/topology.svg[DHCP Server Basic topology]
-endif::testgroup[]
-ifndef::testgroup[]
-image::topology.svg[DHCP Server Basic topology]
-endif::testgroup[]
-endif::topdoc[]
-==== Test sequence
+image::topology.svg[DHCP Server Basic topology, align=center, scaledwidth=75%]
+
+==== Sequence
 . Set up topology and attach to client and server DUTs
 . Configure DHCP server and client
 . Verify DHCP client's original hostname
@@ -27,5 +20,3 @@ endif::topdoc[]
 . Verify DHCP client has no default route
 
 
-<<<
-