Cleaning up CTest tests (#393)

* Improve service startup "tests"
* Address intermittent download test failure
  * Caused by load exceeding threshold
* Add more missing bad HTTPD config tests
* Allow for newer `libpng` with Docker tests
  * Also test more image/file types

docker/centos/stream/Dockerfile

@@ -119,6 +119,7 @@ RUN --mount=id=centos:stream${centos_stream_version}-/var/cache/dnf,target=/var/
         libtiff \
         libwebp \
         make \
+        procps \
         proj
 
 ## Copy files from builder(s)

docker/fedora/Dockerfile

@@ -22,7 +22,8 @@ RUN --mount=id=fedora:${fedora_version}-/var/cache/dnf,target=/var/cache/dnf,typ
       libcurl-devel \
       libmemcached-devel \
       librados-devel \
-      mapnik-devel
+      mapnik-devel \
+      procps
 
 ## Build, Test & Install `mod_tile`
 COPY . /tmp/mod_tile_src