Introduce run-mutation-tests.sh (#170)

This script executes Pitest to determine the code base' mutation test coverage.
The set of tests executed can optionally be restricted by name. The results are 
found in each Maven module's `target/pit-reports` directory.

pom.xml

@@ -73,7 +73,8 @@
         this 'argLine' property instead of the definition of the 'argLine'
         configuration setting allows users or plugins to specify additional
         arguments. In particular, JaCoCo relies on this for the configuration
-        of its Java agent. -->
+        of its Java agent, and Pitest uses this configuration when it spawns
+        additional JVMs. -->
         <argLine>
             <!-- The JVM arguments specified in `.mvn/jvm.config` also apply to
             test JVMs, as those are also non-interactive processess running
@@ -99,7 +100,7 @@
             <!-- We cap memory usage. This may be relevant for build agents,
             but also prevents excessive memory usage by heavily parallelized
             local builds. -->
-            -Xmx1024m
+            -Xmx${argLine.xmx}
             <!-- This argument cannot be set through Surefire's
             'systemPropertyVariables' configuration setting. Setting the file
             encoding is necessary because forked unit test invocations
@@ -118,6 +119,9 @@
             from showing up in the dock and capturing window focus. -->
             -Djava.awt.headless=true
         </argLine>
+        <!-- The maximum amount of heap memory available to forked Surefire
+        processes. Factored out into a separate property to allow overriding. -->
+        <argLine.xmx>1024m</argLine.xmx>
         <!-- Our build system provides a monotonically increasing build number.
         When building locally, this number is obviously absent. So we provide a
         default value. -->

run-mutation-tests.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Executes Pitest to determine the code base' mutation test coverage. The set
+# of tests executed can optionally be restricted by name. The results are found
+# in each Maven module's `target/pit-reports` directory.
+
+set -e -u -o pipefail
+
+if [ "${#}" -gt 1 ]; then
+  echo "Usage: ./$(basename "${0}") [TargetTests]"
+  exit 1
+fi
+
+targetTests=${1:-*}
+
+mvn clean test pitest:mutationCoverage \
+  -DargLine.xmx=2048m \
+  -Dverification.skip \
+  -DfailIfNoTests=false \
+  -Dtest="${targetTests}" \
+  -DtargetTests="${targetTests}"