Handle slash in pkl_test target name

[gordonbondon]

pkl_test_suite produces tests with / in target name, which previously was causing errors like:

An unexpected error has occurred. Would you mind filing a bug report?
     
     java.io.FileNotFoundException: mpackage/path/work/target/name/with_slash_test.xml (No such file or directory)
     	at java.base/java.io.FileOutputStream.open0(Native Method)
     	at java.base/java.io.FileOutputStream.open(FileOutputStream.java:289)
     	at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:230)
     	at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:179)
     	at java.base/java.io.FileWriter.<init>(FileWriter.java:179)
     	at org.pkl.core.stdlib.test.report.TestReport.summarizeToPath(TestReport.java:52)