sh tests: Fix ktrace usage now that envvars are traced by default

Some sh tests use ktrace to see whether a particular file, specified in
the environment, was accessed by the shell. After commit 65a4daeaf324,
this test matches the ktrace record generated by execve.

Use ktrace to only log name lookups, to avoid such false matches.

Fixes: 65a4daeaf324 ("ktrace: log execve(2) arguments and environment")