cosmetics: fix typos and wording in some yacc tests

cosmetics: fix typos and wording in some yacc tests

* tests/yacc-cxx.test (foo.cc): Clarify comment about the content
of this file being valid C++ but invalid C.
(parse1.yy): Likewise.  Also, remove redundant parentheses in a
`return' statement.
* tests/yacc-d-cxx.test (write_parse): Clarify comment about the
content of the generated files being valid C++ but invalid C.
(write_main): Likewise.
* tests/yacc-basic.test: Remove redundant parentheses in a
`return' statement.
* tests/yacc-d-vpath.test: Adjust spacing around curly brackets.
* tests/yaccvpath.test: Likewise.
* tests/yaccdry.test: Likewise.
* tests/yacc8.test: Likewise.
* tests/yacc4.test: Likewise.

Suggested by Ralf Wildenhues.