title: Why is the JaCoCo .exec file generated for Android off-device tests almost empty if testCoverageEnabled is true, but only for library modules?
body: <p>I'm working in a multi-module Android project with an application module and a library module. If I simply add</p>
  <pre><code>apply plugin: 'jacoco'
  jacoco {
       toolVersion '0.8.7'
  }
  </code></pre>
  <p>to the <code>build.gradle</code> for each module, then when I run the off-device unit tests from the command line in each module's directory, I see an .exec file created at <code>build/jacoco/testDebugUnitTest.exec</code> with a size of ~27k or so. Generating a report from this does what I'd expect.</p>
  <p>If I add</p>
  <pre><code>buildTypes {
      debug {
          testCoverageEnabled true
      }
  }
  </code></pre>
  <p>to the build.gradle for each module (so that I can later gather coverage numbers for on-device tests as well) then when I run the off-device unit tests the .exec file is instead created in <code>build/outputs/unit_test_code_coverage/debugUnitTest/testDebugUnitTest.exec</code>.</p>
  <p>More importantly, while that file is still ~27k when I'm running the tests in the application module, it is almost empty (only 45 bytes) when I'm running the tests in a library module in the same project.</p>
  <p>Is there a reason testCoverageEnabled interferes with off-device code coverage for library modules but not for the primary application module?</p>