Optimization flags are removed for all tests

`CMakeLists.txt` in `suite/tests` has the line
`string(REGEX REPLACE "-O[0-9]? " " " CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")`
which strips out any optimization flags for the tests. Optimization is then selectively added later, using the `optimize()` or `add_sve_flags()` functions.

A better approach would be to allow optimization by default, and strip optimization flags from those tests that cannot support them. This was investigated and proved to be non-trivial as the client tests use `_DR_set_compile_flags()` to set the compile flags whereas the core tests use `set_cflags()`. `_DR_set_compile_flags()` is a public function and we didn't want to add a blocklist in there.

This issue covers the work to remove the blanket removal of optimization flags and find a way to strip optimization flags from tests that cannot support them, both for core and client tests.