silent-rules tests: fix spurious failures with Sun Studio C++

silent-rules tests: fix spurious failures with Sun Studio C++

* tests/silentcxx.test: The C++ compiler from Sun Studio is named
`CC'.  Account for this in our grepping checks on the make output.
Since we are at it, throw in a couple of improvements to comments
and formatting.
* tests/silent-many-generic.test: The C++ compiler from Sun Studio
is named `CC', and this can cause spurious failures in our grepping
of the make output.  Work around this by using a wrapper script
around the C++ compiler (generated on the fly), since filtering the
make output proved to be too fragile.