test_coverage: calculate the difference in reverse

If we increase the coverage the message prints a negative value
as the difference from the previous value. This can be confusing
as it looks like the coverage has decreased.

For example if the value in coverage_config_x86_64.json is 73.42,
and the new coverage is 73.96 (increased), we have the following
error:

    ValueError: Current code coverage (73.96%) deviates by -0.54%
    from the previous code co...

Let's calculate the difference in reverse so that we have a
negative value if it decreases and positive otherwise.

Signed-off-by: Stefano Garzarella <[email protected]>

integration_tests/test_coverage.py

@@ -116,7 +116,7 @@ def test_coverage(profile, no_cleanup, test_scope):
     coverage_config = _read_test_config()
     current_coverage = _get_current_coverage(coverage_config, no_cleanup, test_scope)
     previous_coverage = coverage_config["coverage_score"]
-    diff = previous_coverage - current_coverage
+    diff = current_coverage - previous_coverage
     upper = previous_coverage + MAX_DELTA
     arch = platform.machine()