Split tests into two packages in Bazel build.

[ftynse]

This separates MLIR tests and regular Enzyme integration tests into independent packages, making sure only a subset of targets is rebuilt and retested when requested to decrease overhead.

[wsmoses]

ERROR: no such package 'third_party/enzyme': BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
 - /home/runner/work/Enzyme/Enzyme/enzyme/third_party/enzyme
ERROR: /home/runner/work/Enzyme/Enzyme/enzyme/test/Integration/BUILD:8:13: every rule of type py_test implicitly depends upon the target '//third_party/enzyme:license', but this target could not be found because of: no such package 'third_party/enzyme': BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
 - /home/runner/work/Enzyme/Enzyme/enzyme/third_party/enzyme
Analyzing: [18](https://github.com/EnzymeAD/Enzyme/actions/runs/7962983772/job/21737646771?pr=1741#step:6:19)3 targets (73 packages loaded, 788 targets configured)
[2 / 2] checking cached actions
Analyzing: 183 targets (73 packages loaded, 788 targets configured)
[2 / 2] checking cached actions
Target //test:lit_site_cfg_py up-to-date:
  bazel-bin/test/lit.site.cfg.py
ERROR: Analysis of target '//test/Integration:ReverseMode/taylorlog.c.test' failed; build aborted: Analysis failed
INFO: Elapsed time: 7.078s, Critical Path: 0.01s
INFO: 2 processes: 2 internal.
ERROR: Build did NOT complete successfully
ERROR: No test targets were found, yet testing was requested