tests: rename $am_make_rc_got -> $am_make_rc

tests: rename $am_make_rc_got -> $am_make_rc

The latter is more natural  More importantly, I keep misspelling
the former over and over.

* t/ax/am-test-lib.sh (run_make): Update.
* t/ax/tap-summary-aux.sh: Adjust.
* t/ax/testsuite-summary-checks.sh: Likewise.
* t/parallel-tests-exit-statuses.sh: Likewise.
* t/parallel-tests-extra-programs.sh: Likewise.
* t/parallel-tests-fd-redirect-exeext.sh: Likewise.
* t/parallel-tests-fd-redirect.sh: Likewise.
* t/parallel-tests-fork-bomb.sh: Likewise.
* t/parallel-tests-no-spurious-summary.sh: Likewise.
* t/parallel-tests-recheck-pr11791.sh: Likewise.
* t/parallel-tests-reset-term.sh: Likewise.
* t/tap-signal.tap: Likewise.
* t/test-driver-acsubst.sh: Likewise.
* t/test-driver-cond.sh: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>