Fix: tests: print real values in a fixed format

Fix: tests: print real values in a fixed format

The cli_params_to_string.to_string function converts float values
(bt2._RealValueConst) to string using the str function.  The format of
that seems to differ across platform.  On a 32-bit powerpcspe platform,
we see this failure in the cli/params/test_params test:

  -{a=10500000.0, b=10500000.0, c=1.05e-05, d=1.05e-05}
  +{a=10500000.0, b=10500000.0, c=1.0499999999999999e-05, d=1.0499999999999999e-05}

Fix that by doing an explicit string conversion for _RealValueConst objects,
with a constant 7 decimal places after the decimal point.  The 7 is chosen so
that the numbers in tests in cli/params/test_params are not truncated.

Change-Id: Ie4fcb8eb422c3d44eeec1201169dc8a995536d4a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9651
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>