tests: improve `ccnoco*.test', better cross-compiling support

tests: improve `ccnoco*.test', better cross-compiling support

* tests/ccnoco3.test (Mycomp): Use the `$CC' chosen by `tests/defs'
instead of forcing `gcc' unconditionally.  This ensures better
coverage in case of cross-compiling, when GCC can be named e.g.,
`i586-mingw32msvc-gcc', instead of simply `gcc'.
* tests/ccnoco.test: Likewise.  Remove redundant checks.  Modernize
the created `configure.in'.  Run tests both in-tree and in VPATH.
Export `CC' to the overridden value only once.
* tests/ccnoco2.test: Slightly stricter grepping of automake
stderr.  Add trailing `:' command.