"polish" test code (minor update)

    * tests/test-breakpoint.cc: Remove code to silence unused function
    parameter compiler warning - use unnamed function parameters
    instead.
    * tests/test-core.cc: Likewise.
    * tests/test-cpp-lexer.cc: Likewise.
    * tests/test-cpp-parser.cc: Likewise.
    * tests/test-deref.cc: Likewise.
    * tests/test-gdbmi.cc: Likewise.
    * tests/test-global-variables.cc: Likewise.
    * tests/test-libtool-wrapper-detection.cc: Likewise.
    * tests/test-local-vars-list.cc: Likewise.
    * tests/test-overloads.cc: Likewise.
    * tests/test-unicode.cc: Likewise.
    * tests/test-var-list.cc: Likewise.
    * tests/test-var-path-expr.cc: Likewise.
    * tests/test-var-walker.cc: Likewise.
    * tests/test-watchpoint.cc: Likewise.