tests/test_non_interactive.R: Break up example script into multiple lines
R/non_interactive_summary.R: Remove broken detection for tests ending prematurely

When using tryCatch(), any caught error condition will remain in the
geterrmessage() buffer. As a result, any code that uses tryCatch() will produce
spurious "script ended prematurely" messages.

If the test genuinely fails prematurely, then the exit status will be non-zero
so the error will be detected by "CMD check".