Use default workingDir for tests

[devinrsmith]

We've been unable to upload JVM err logs due to the upload action being unable to find the files.

The GH working directory logs:

Working directory is '/home/runner/work/deephaven-core/deephaven-core'

SIGSEGV logs:

# An error report file with more information is saved as:
# /home/runner/work/deephaven-core/hs_err_pid9554.log

Turns out we are settings the workingDir for tests as the parent of the root dir; instead of leaving it as the project dir:

    p.tasks.withType(Test).all { Test t -> t.with {
        workingDir = "$rootDir/.."

This is likely due to historical concerns where we were checking out multiple repositories.

We should go back to the standard working directories in an effort to reduce complexity, as well as allowing the uploader action to find the JVM err logs against its default working directory.