Assert no spaces in crate features JSON

Spaces in the `crate_features` JSON value can cause errors when running
kcov.

Fixes: https://github.com/rust-vmm/rust-vmm-ci/issues/74

Signed-off-by: Ahmed Abouzied <email@aabouzied.com>

integration_tests/test_coverage.py

@@ -51,6 +51,9 @@ def _read_test_config():
     assert "exclude_path" in coverage_config
     assert "crate_features" in coverage_config
 
+    assert ' ' not in coverage_config["crate_features"], \
+        "spaces are not allowed in crate_features value"
+
     return coverage_config