tests: move tests/fixtures -> test/testdata, add test_linechart_timeseries

plot {

  axis-top: off;

  axis-right: off;

  series {

    xs: 10   20   30   40   50   60    70   80  90  100;

    ys: 1.23 4.32 3.23 6.43 3.45 12.32 8.14 5.2 3.5 2.2;

  }

}