Run tests with coverage

- Use jacoco for Java-side coverage. Our Java coverage is /terrible
   (~10%), as we only really test the core libraries. Still a good thing
   to track for regressions though.

 - mcfly now tracks Lua side coverage. This works in several stages:
   - Replace loadfile to include the whole path
   - Add a debug hook which just tracks filename->(lines->count). This
     is then submitted to the Java test runner.
   - On test completion, we emit a luacov.report.out file.

   As the debug hook is inserted by mcfly, this does not include any
   computer startup (such as loading apis, or the root of bios.lua),
   despite they're executed.

   This would be possible to do (for instance, inject a custom header
   into bios.lua). However, we're not actually testing any of the
   behaviour of startup (aside from "does it not crash"), so I'm not
   sure whether to include it or not. Something I'll most likely
   re-evaluate.

build.gradle

@@ -18,6 +18,7 @@ buildscript {
 
 plugins {
     id "checkstyle"
+    id "jacoco"
     id "com.github.hierynomus.license" version "0.15.0"
     id "com.matthewprenger.cursegradle" version "1.3.0"
     id "com.github.breadmoirai.github-release" version "2.2.4"
@@ -251,6 +252,15 @@ test {
     }
 }
 
+jacocoTestReport {
+    reports {
+        xml.enabled true
+        html.enabled true
+    }
+}
+
+check.dependsOn jacocoTestReport
+
 license {
     mapping("java", "SLASHSTAR_STYLE")
     strictCheck true