tests: avoid two false-positive parallel test failures

* tests/local.mk (check-local): Add a leading "+",
to void false-positive test failures when running
them in parallel.  Before this change, running e.g.,
"make check TESTSUITEFLAGS=--jobs=15" would always
fail the two "make"-invoking tests: "C unit tests"
and "C unit tests (EXEEXT)".
(installcheck-local): Likewise for "installcheck".