Performance variables are not stored in run report

[vkarak]

This is an excerpt from a report of a test that uses the new performance variable syntax:

        {
          "build_stderr": "rfm_StridedBandwidthTest_128_build.err",
          "build_stdout": "rfm_StridedBandwidthTest_128_build.out",
          "dependencies_actual": [],
          "dependencies_conceptual": [],
          "description": "StridedBandwidthTest_128",
          "prefix": "/users/karakasv/Devel/reframe/cscs-checks/microbenchmarks/cpu/strided_bandwidth",
          "filename": "/users/karakasv/Devel/reframe/cscs-checks/microbenchmarks/cpu/strided_bandwidth/strides.py",
          "environment": "PrgEnv-gnu",
          "fail_phase": null,
          "fail_reason": null,
          "jobid": "1562263",
          "job_stderr": "rfm_StridedBandwidthTest_128_job.err",
          "job_stdout": "rfm_StridedBandwidthTest_128_job.out",
          "maintainers": [
            "SK"
          ],
          "name": "StridedBandwidthTest_128",
          "nodelist": [
            "nid00407"
          ],
          "outputdir": "/users/karakasv/Devel/reframe/output/dom/mc/PrgEnv-gnu/StridedBandwidthTest_128",
          "perfvars": null,
          "result": "success",
          "stagedir": "/users/karakasv/Devel/reframe/stage/dom/mc/PrgEnv-gnu/StridedBandwidthTest_128",
          "scheduler": "slurm",
          "system": "dom:mc",
          "tags": [
            "diagnostic",
            "benchmark"
          ],
          "time_compile": 2.29341459274292,
          "time_performance": 0.008340835571289062,
          "time_run": 47.37093162536621,
          "time_sanity": 0.004153251647949219,
          "time_setup": 0.010731220245361328,
          "time_total": 49.688939571380615
        }

Notice how perfvars are reported as null.

[teojgo]

Fixed by #2209