Record skipped test as skipped in testsuite summary

When system test execution was ported to Automake, SKIPPED and UNTESTED system test result were not made to match Automake expectations, therefore a skipped test is recorded by Automake as "PASS":

$ make check TESTS=cpu V=1
I:cpu:cpu test only runs on Linux, skipping test
I:cpu:Prerequisites missing, skipping test.
R:cpu:SKIPPED
E:cpu:2020-12-16T11:36:58+0000
PASS: cpu
====================================================================
Testsuite summary for BIND 9.17.7
====================================================================
# TOTAL: 1
# PASS:  1

For a test to be recorded by Automake as skipped, the test, or it's test driver, needs to exit with code 77:

$ make check TESTS=cpu V=1
I:cpu:cpu test only runs on Linux, skipping test
I:cpu:Prerequisites missing, skipping test.
R:cpu:SKIPPED
E:cpu:2020-12-16T11:39:10+0000
SKIP: cpu
====================================================================
Testsuite summary for BIND 9.17.7
====================================================================
# TOTAL: 1
# PASS:  0
# SKIP:  1