parse-datetime, tests: don't use "string" + int

parse-datetime, tests: don't use "string" + int

Recent versions of 'clang' complain about C source code that
uses expressions of the form '"string literal" + integer',
I guess on the theory that it's confusing for readers who are
used to C++.  On those grounds I suppose it's OK to make this
minor style change.
* lib/parse-datetime.y (parse_datetime):
* tests/test-fchdir.c (main):
* tests/test-snprintf-posix.h (test_function):
* tests/test-snprintf.c (main):
* tests/test-vasnprintf-posix.c (test_function):
* tests/test-vasnprintf.c (test_function):
* tests/test-vsnprintf.c (main):
* tests/unistdio/test-ulc-asnprintf1.h (test_function):
Rewrite '"str" + E' to '&"str"[E]'.