Add possibility to run complement with coverage enabled (#2901)

This adds the possibility to run Complement with coverage enabled.
In combination with https://github.com/matrix-org/complement/pull/566 we
should then be able to extract the coverage logs, combine them with
https://github.com/wadey/gocovmerge (or similar) and upload them to
Codecov (with different flags, depending on SQLite, HTTP etc.)

build/scripts/complement-cmd.sh

@@ -0,0 +1,22 @@
+#!/bin/bash -e
+
+# This script is intended to be used inside a docker container for Complement
+
+if [[ "${COVER}" -eq 1 ]]; then
+  echo "Running with coverage"
+  exec /dendrite/dendrite-monolith-server-cover \
+    --really-enable-open-registration \
+    --tls-cert server.crt \
+    --tls-key server.key \
+    --config dendrite.yaml \
+    -api=${API:-0} \
+    --test.coverprofile=integrationcover.log
+else
+  echo "Not running with coverage"
+  exec /dendrite/dendrite-monolith-server \
+    --really-enable-open-registration \
+    --tls-cert server.crt \
+    --tls-key server.key \
+    --config dendrite.yaml \
+    -api=${API:-0}
+fi