openpower-pels: clean up various compile warnings

Compile warnings observed when compiling parts of the
openpower-pels (or corresponding tests) under stricter
compiler warning flags of Meson.

Issues fixed:
    - many unused parameters
    - invalid case fall-through
    - excess semi-colons
    - incorrect 'const' on return-by-value type
    - removal of variable length array in test case
    - uncaught return from 'system' call in test case

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8af69184042cf8661d1307a02ecf3afcab4724a1
diff --git a/test/openpower-pels/mocks.hpp b/test/openpower-pels/mocks.hpp
index a2c3b5a..a6a2665 100644
--- a/test/openpower-pels/mocks.hpp
+++ b/test/openpower-pels/mocks.hpp
@@ -202,7 +202,7 @@
      * @param[in] fd - The file descriptor used
      * @param[in] events - The event bits
      */
-    void receive(sdeventplus::source::IO& source, int fd,
+    void receive(sdeventplus::source::IO& /*source*/, int /*fd*/,
                  uint32_t events) override
     {
         if (!(events & EPOLLIN))