tests: implement strutil tests for text line and word splitting

tests: implement strutil tests for text line and word splitting

Add the test_text_line() and test_text_word() routines to cover the
splitting of text buffers into lines and the splitting of words on a
text line.

This implementation "suffers" from global vars at the file scope,
because moving the table of test cases into the test routine results in
compiler errors (initializer data not constant), the cause is uncertain.