Reapply "Fix init_script_test_helper not being cleaned properly."

Looks like 520144f was an attempt to
fix init_script_test_helper from not being cleaned properly. This commit
was reverted in ebd97bd, and another
attempt was added in 9dc759f. However,
the revert actually removed the entire `init_script_test_helper` binary
from being compiled. This reapplying the first fix, and combining it
with the second fix should do the trick.

Found this while trying to clear the warning:
```
tests/unit/Makefile.am:402: warning: variable 'init_script_test_helper_SOURCES' is defined but no program or
tests/unit/Makefile.am:402: library has 'init_script_test_helper' as canonical name (possible typo)
```

This reverts commit ebd97bd.

Author: larsewi

tests/unit/Makefile.am

@@ -407,6 +407,7 @@ nfs_test_LDADD = ../../libpromises/libpromises.la libtest.la
 init_script_test_helper_SOURCES = init_script_test_helper.c
 init_script_test.sh: init_script_test_helper
 CLEANFILES += init_script_test_helper
+noinst_PROGRAMS += init_script_test_helper
 endif
 EXTRA_DIST += init_script_test_helper.c
 EXTRA_DIST += init_script_test.sh

tests/unit/init_script_test.sh

@@ -91,10 +91,10 @@ fi
 # Fail on any error.
 set -e
 
-cp init_script_test_helper $CFTEST_PREFIX/bin/cf-test-execd
-cp init_script_test_helper $CFTEST_PREFIX/bin/cf-test-serverd
-cp init_script_test_helper $CFTEST_PREFIX/bin/cf-test-monitord
-cp init_script_test_helper $CFTEST_PREFIX/bin/cf-test-agent
+cp .libs/init_script_test_helper $CFTEST_PREFIX/bin/cf-test-execd
+cp .libs/init_script_test_helper $CFTEST_PREFIX/bin/cf-test-serverd
+cp .libs/init_script_test_helper $CFTEST_PREFIX/bin/cf-test-monitord
+cp .libs/init_script_test_helper $CFTEST_PREFIX/bin/cf-test-agent
 
 touch $CFTEST_PREFIX/inputs/promises.cf