tests: Fix GCC -W(maybe-)uninitialized warnings

These all look like valid (but harmless) diagnostics to me and are
all simple to fix. Most of them can be fixed by using ASSERT_* variants
of the GTest checkers to ensure that the remainder of the function is
not executed and the uninitialized result isn't touched.

Change-Id: Ib5fe2ac2ec539c880d670ebc3321ce98940c7e38
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/68517
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/base/circlebuf.test.cc

@@ -136,7 +136,7 @@ TEST(CircleBufTest, PointerWrapAround)
 TEST(CircleBufTest, ProduceConsumeEmpty)
 {
     CircleBuf<char> buf(8);
-    char foo[1];
+    char foo[1] = {'a'};
 
     // buf is empty to begin with.
     EXPECT_TRUE(buf.empty());