Wrapper script does not work with --singlejar

If using the `--singlejar` option the wrapper script does not work.

Expected: should work as specified for java (https://docs.bazel.build/versions/master/tutorial/java.html#package-a-java-target-for-deployment)

The line `JAVABIN=${TEST_SRCDIR}/__main__/app/app_wrapper.sh` is "evaluated" to `/__main__/app/app_wrapper.sh` which is invalid.

With singlejar flag:
```
$ ./bazel-bin/app/app --singlejar --print_javabin
/__main__/app/app_wrapper.sh
```

Without singlejar flag:
```
$ ./bazel-bin/app/app --print_javabin
/Users/xxx/code/bazeltest/project/./bazel-bin/app/app.runfiles/__main__/app/app_wrapper.sh
```
There should probably be an option to override JAVABIN path like in the java case:
```
JAVABIN=${JAVABIN:-${JAVA_RUNFILES}/local_jdk/bin/java}
```

Currently the wrapper script hardcodes it to `${TEST_SRCDIR}/__main__/../local_jdk/bin/java`.

Start with app/app_wrapper.sh and the --singlejar and --print_javabin entry points described in the report. Compare the singlejar wrapper's JAVABIN handling with the Java behavior referenced in the issue; done means the wrapper works with --singlejar and prints a valid JAVABIN path.

Written by the indexing model from the issue text.