Add a test checking that the One-JAR version can be overridden

[stigkj]

Add a test checking that overriding which version of One-JAR to use actually works. It would look like this:

dependencies {
    ...
    executableJar 'one-jar:one-jar-boot:0.96'
    ...
}

[stigkj]

Not a proper unit test, but the sample build :execjar1 overrides it, check out commit 2cc429e4b6036e213b64a5e9885cd42521bf5c26.