THRIFT-5768 Add missing test in configure.ac for kotlin

The Kotlin autoconfig script was missing a `test` causing it to try and
execute `x/usr/local/bin/gradle` instead of test for its existence. This
resulted in the following error:
```
./configure: line 15049: x/usr/local/bin/gradle: No such file or directory
```

Adding `test` results in the configuration succeeding. Configure output
now:
```
thrift 0.21.0

Building C (GLib) Library .... : yes
Building C++ Library ......... : yes
Building Common Lisp Library.. : yes
Building D Library ........... : yes
Building Dart Library ........ : yes
Building .NET Standard Library : yes
Building Erlang Library ...... : yes
Building Go Library .......... : yes
Building Haxe Library ........ : yes
Building Java Library ........ : yes
Building Kotlin Library ...... : yes
Building Lua Library ......... : yes
Building NodeJS Library ...... : yes
Building Perl Library ........ : yes
Building PHP Library ......... : yes
Building Python Library ...... : yes
Building Py3 Library ......... : yes
Building Ruby Library ........ : yes
Building Rust Library ........ : yes
Building Swift Library ....... : yes
```