Failing functional tests stop lcov

Summary:

Functional tests can fail and lcov still generates a coverage file,
which is inaccurate. This change stops make from proceeding if
functional tests fail.

Backport of core PR16207.

Test Plan:

../configure --enable-lcov
make

Edit a functional test so that it will fail.

make cov

Check the coverage report process is aborted.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5302