Create parent dirs to JUnit report file if needed

[aengelberg]

To make the JUnit reporting feature more easily useable in CI workflows, I propose automatically creating parent directories of the JUnit report file if needed. This allows CircleCI config like the following:

- run: lein run -m my.main.greenlight.ns --junit-report build/junit.xml
- store_test_results:
    path: build

This change also makes Greenlight's JUnit reporter more consistent with other JUnit reporters like kaocha and test2junit.

[codecov-commenter]

Codecov Report

Merging #54 (d49ea45) into master (b06c59c) will increase coverage by 0.96%.
The diff coverage is 100.00%.

❗ Current head d49ea45 differs from pull request most recent head 65220b1. Consider uploading reports for the commit 65220b1 to get more accurate results

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   66.26%   67.23%   +0.96%     
==========================================
  Files           6        6              
  Lines         584      586       +2     
  Branches       39       38       -1     
==========================================
+ Hits          387      394       +7     
+ Misses        158      154       -4     
+ Partials       39       38       -1     

Impacted Files	Coverage Δ
src/greenlight/report.clj	78.43% <100.00%> (+1.43%)	⬆️
src/greenlight/runner.clj	40.11% <0.00%> (+1.69%)	⬆️
src/greenlight/report/junit.clj	97.05% <0.00%> (+2.94%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b06c59c...65220b1. Read the comment docs.

[greglook]

LGTM, nice quality-of-life improvement. 🙆‍♂️

[drewinglis]

Looks like we need to update cljstyle.