doc: fix missing images in test system arch.

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

doc/test-arch.md

@@ -108,7 +108,7 @@ DHCP server, and the other two as DHCP clients - with all three having
 a management connection to the host PC running the test.  In other
 words, the test requires a _logical_ topology like the one below.
 
-<img align="right" src="img/testing-log.dot.svg" alt="Example Logical Topology">
+![Example Logical Topology](img/testing-log.svg){ align=right width="360" }
 
 ```dot
 graph "dhcp-client-server" {
@@ -155,7 +155,7 @@ degrees), we can deploy well-known algorithms to find such subgraphs.
 Continuing our example, let's say we want to run our DHCP test on the
 _physical_ topology below.
 
-<img align="right" src="img/testing-phy.dot.svg" alt="Example Physical Topology">
+![Example Physical Topology](img/testing-phy.svg){ align=right width="360" }
 
 ```dot
 graph "quad-ring" {