Silence assert redefinition warning

[laustam]

Currently, every test run using make (for example: make testall) causes two compilation warnings. These occur due to the redefinition of the assert macro. On my machine (Mac M1), these warnings look something like this:

tests/testutil.h:62:9: warning: '__assert' macro redefined [-Wmacro-redefined]
#define __assert(c) do { \
        ^

tests/testutil.h:70:9: warning: 'assert' macro redefined [-Wmacro-redefined]
#define assert(c) __assert(c);
        ^

Since these warnings do not indicate anything about students' code, I suggest that we silence these so as not to confuse students. This can be done by using the #pragma directive, as implemented in this PR.

Here is a short description of each of the #pragma directives used in this PR:

• #pragma clang diagnostic push: This directive saves the current state of all diagnostic settings. It's used to create a stack of diagnostic states so that temporary changes can be made to the diagnostic settings and then revert back to the previous state.

• #pragma clang diagnostic ignored "-Wmacro-redefined": This directive tells the Clang compiler to ignore the specific warning identified by -Wmacro-redefined. This is exactly what is triggered by the assert redefinition, so we want to silence this.

• #pragma clang diagnostic pop: After the block of code where the redefinition warning is suppressed, this directive restores the previous state of the diagnostic settings that were saved by #pragma clang diagnostic push. This ensures that any changes to the diagnostic settings are only temporary and do not affect the rest of the code.