test: fix sed error in basic tests

test: fix sed error in basic tests

The error is easy to miss, because the test passes and stderr is not
printed.  But if you run basic tests in verbose mode (./basic
--verbose), you get:

  sed: can't read notmuch-test: No such file or directory

The issue is that sed command is given two files: notmuch-test and
$TEST_DIRECTORY/notmuch-test.  And there is no notmuch-test file in
the current directory (test/tmp.basic/).  The patch just removes the
non-existing file from the sed command.