Add stripped unit tests for decoder

Add stripped unit tests for decoder

That is, test cases where there's no newline or other whitespace at
the beginning or end of input. This was implemented by adding a
--strip option to split-testfile to strip the input file after writing
it.

The actual test JSON texts are the same as testdata/invalid and
testdata/valid. The expected output of the invalid cases had to be
adjusted a bit: because there's no newline at the end, some of the
line numbers needed to be changed.