[CALCITE-3888] Switch avatica-server to be test dependency for core
[CALCITE-3888] Switch avatica-server to be test dependency for core

`avatica-server` was a test only dependencies for `calcite-core`.
But during the switch to gradle in 1.22.0, it's configured to be an
implementation dependency.

This PR switches it back to testImplementation only.

apache/calcite,googleinterns/calcite,jcamachor/calcite,looker-open-source/calcite,vlsi/calcite,vlsi/incubator-calcite,jcamachor/calcite,julianhyde/calcite,julianhyde/calcite,julianhyde/calcite,vlsi/calcite,googleinterns/calcite,julianhyde/calcite,looker-open-source/calcite,googleinterns/calcite,vlsi/calcite,googleinterns/calcite,julianhyde/calcite,jcamachor/calcite,apache/calcite,jcamachor/calcite,looker-open-source/calcite,vlsi/calcite,julianhyde/calcite,looker-open-source/calcite,vlsi/calcite,jcamachor/calcite,vlsi/incubator-calcite,looker-open-source/calcite,googleinterns/calcite,apache/calcite,apache/calcite,jcamachor/calcite,vlsi/calcite,apache/calcite,apache/calcite,looker-open-source/calcite,googleinterns/calcite