pixelstats: MM Metrics 2022: Add test code/data

* Implements the derived class for MmMetricsReporter class to redirect
  Sysfs node reads to the test patterned data files.

* Add test code using the derived class for both atoms.

* Test theory:
    - Test data carefully crafted to make all output numbers unique.
    - The test aims to catch write offset bug:
        + One atom might be prepared (the data array) by multiple
          functions.
        + If they write to the wrong offsets, the test will catch
          the errors.
    - By comparing with the golden test results it also proves the
      correctness.

* Covered MM atoms:
    - PixelMmMetricsPerHour
    - PixelMmMetricsPerDay

* MM atoms not covered, (i.e. older atoms)
    - ZramBdStat
    - ZramMmStat
    - CmaStatus
    - CmaStatusExt

Bug: 244671514
Test: atest pixelstats_mm_test
      Note: known bugs expected. Will failed the last test case.
            Fixes will be in the later commits.

Change-Id: I5c87ce6f59e9589a913ae3703d250604ee2fda75
Signed-off-by: Robin Hsu <robinhsu@google.com>