[Jacoco] Allow skipping modules that have no tests

[tlinkowski]

In my little project, I have a test module (unij-test) with no tests whatsoever (the module just provides shared test code for other modules).

When I run standard jacocoTestReport for unij-test, it gets SKIPPED.

However, when I apply org.kordamp.gradle.jacoco, and then run its jacocoRootReport, I get the following error:

Unable to read [path]\UniJ\subprojects\unij-test\build\jacoco\test.exec

I'm not sure if this is intentional or not, but I think that it could be useful to have some way (either implicit or explicit) of skipping a module in JaCoCo merging/reporting.

PS. Note that, after all, I haven't used this plugin, because I realized that a special codecov uploader can also do the report assembling for me.

[aalmiray]

Here's one way to conditionally skip running JaCoCo on a project that does not define tests under `src/test'

jacoco {
    enabled = subproject.file('src/test').exists()
}

Take note that JaCoCo will be applied to all test tasks, including those defined by integration-test and functional-test plugins.

[tlinkowski]

Yes, good point - if my library were more complex and I needed integration/functional tests, then this plugin would come in very handy