tests: changed coverage file name

The latest version of kcov no longer appears to write the coverage info
we're looking for in a file named 'index.json'. A new file `index.js`
seems to have taken its place so we updated the appropriate constant
(KCOV_COVERAGE_FILE) in test_coverage.py from the build tests folder.

Signed-off-by: Alexandru Agache <[email protected]>

Author: alexandruag

tests/integration_tests/build/test_coverage.py

@@ -22,7 +22,7 @@
 
 CARGO_KCOV_REL_PATH = os.path.join(host.CARGO_BUILD_REL_PATH, 'kcov')
 
-KCOV_COVERAGE_FILE = 'index.json'
+KCOV_COVERAGE_FILE = 'index.js'
 """kcov will aggregate coverage data in this file."""
 
 KCOV_COVERAGE_REGEX = r'"covered":"(\d+\.\d)"'