Pass coverage config file by absolute path

The kernel may not be running in the same working directory as the test
process.

Author: takluyver

.gitignore

@@ -6,3 +6,4 @@ __pycache__
 *.egg-info
 .cache
 .doit.db*
+.coverage*

nbval/cover.py

@@ -51,6 +51,8 @@ def setup_coverage(config, kernel, floc, output_loc=None):
         # Get options from pytest-cov's command line arguments:
         source = config.option.cov_source
         config_file = config.option.cov_config
+        if config_file and isinstance(config_file, str):
+            config_file = os.path.abspath(config_file)
 
         # Copy the suffix of plugin if available
         suffix = _make_suffix(cov)