Create template for tests

[albinahlback]

Tests looks very similar, like

#include "fmpz.h"

int main(void)
{
    /* Declarations */

    FLINT_TEST_INIT(state);
    flint_printf("add....");
    fflush(stdout);

    /* Performing tests */

    FLINT_TEST_CLEANUP(state);
    flint_printf("PASS\n");
    return 0;
}

We can reduce this, just like what is currently proposed in #1341, to something like

#include "test_helpers.h"
#include "fmpz.h"

TEST_FUNCTION_START(fmpz_add)
{
    /* Declarations */

    /* Performing tests */

    TEST_FUNCTION_END;
}

Note: I want to add that I think it is a good idea to print fmpz_add... instead of add... since it clearly specifies what function is being tested.

[albinahlback]

Completed as of #1341.