CI: improve gcov handling #10404

- Move __gcov_flush to process_spawn, for more reliable coverage
  tracking of subprocesses
- Travis: use GCOV_ERROR_FILE
- codecov: use "-X fix" to skip "fixing" uploaded coverage data; it
  should be handled by codecov's backend instead.
- AppVeyor: no $PATH mangling, which breaks with the improved coverage tracking
  due to missing .dll in PATH.

ci/script.sh

@@ -11,3 +11,9 @@ if [[ "${TRAVIS_OS_NAME}" == osx ]]; then
 else
   ci/run_${CI_TARGET}.sh
 fi
+
+if [[ -s "${GCOV_ERROR_FILE}" ]]; then
+  echo '=== Unexpected gcov errors: ==='
+  cat "${GCOV_ERROR_FILE}"
+  exit 1
+fi