Allow for enabling data builders only in test flavours

[demoritas]

Use case

Currently

 generateDataBuilders.set(true)

can only be globally enabled, this leads to data builders ending up in production code which is not optimal.

Describe the solution you'd like

It would be great to have a way to only enable them for tests

[zaenk]

this used to be a feature for testDataBuilders

generateTestBuilders.set(true)
testDirConnection {
    connectToKotlinSourceSet("testFixtures")
}

testDataBuilders is now deprecated in favor of generateDataBuilders - but according to the v4 migration guides theres noting to replace it with

• v3

  apollo-kotlin/intellij-plugin/src/test/testData/migration/v3-to-v4/gradleDeprecations.gradle.kts

  Line 10 in 542fc1f

  testDirConnection {

• v4 https://github.com/apollographql/apollo-kotlin/blob/542fc1faab16553335a888789cb6290277fe53c8/intellij-plugin/src/test/testData/migration/v3-to-v4/gradleDeprecations_after.gradle.kts