test: add test samples

Adds test samples that can be used for unit tests or to verify
implementations via:
```bash
for sample in $(ls src/test/resources/samples/*.txt)
do
  echo "Validating $sample"
  rm -f measurements.txt
  ln -s $sample measurements.txt

  diff <(./calculate_average.sh) ${sample%.txt}.out
done
rm measurements.txt
```

For #61

.gitignore

@@ -41,4 +41,4 @@ nb-configuration.xml
 #JReleaser
 out/
 
-measurements*.txt
+/measurements*.txt