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.

test/test-valid

@@ -5,7 +5,7 @@
 # Jansson is free software; you can redistribute it and/or modify
 # it under the terms of the MIT license. See LICENSE for details.
 
-TESTFILES="${srcdir}/testdata/valid"
+TESTFILES="${srcdir}/testdata/valid ${srcdir}/testdata/valid-strip"
 
 run_test() {
     local prog=$1