Tests: static vs non-static constexpr variables

Problem:
- `static` variables consume memory and sometimes are less
  optimizable.
- `static const` variables can be `constexpr`, usually.
- `static` function-local variables require an initialization check
  every time the function is run.

Solution:
- If a global `static` variable is only used in a single function then
  move it into the function and make it non-`static` and `constexpr`.
- Make all global `static` variables `constexpr` instead of `const`.
- Change function-local `static const[expr]` variables to be just
  `constexpr`.

Tests/LibC/snprintf-correctness.cpp

@@ -42,10 +42,10 @@ static String show(const ByteBuffer& buf)
     return builder.build();
 }
 
-static const size_t SANDBOX_CANARY_SIZE = 8;
-
 static bool test_single(const Testcase& testcase)
 {
+    constexpr size_t SANDBOX_CANARY_SIZE = 8;
+
     // Preconditions:
     if (testcase.dest_n != testcase.dest_expected_n) {
         warnln("dest length {} != expected dest length {}? Check testcase! (Probably miscounted.)", testcase.dest_n, testcase.dest_expected_n);