Save the coverage report to a file

Save the "Test Report Summary" to a file. This can help both CI scripts and
human readers who want the summary after the fact without having to copy the
console output.

Take care to exit with a nonzero status if there is a failure while
generating the test report summary.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.gitignore

@@ -18,6 +18,7 @@ Testing
 Coverage
 *.gcno
 *.gcda
+coverage-summary.txt
 
 # generated by scripts/memory.sh
 massif-*