TESTS: Add -std=gnu99 to cwrap tests CFLAGS
    
    ../../../../src/tests/cwrap/../../../src/util/domain_info_utils.c: In function ‘subdomain_enumerates’:
    ../../../../src/tests/cwrap/../../../src/util/domain_info_utils.c:77:9: error: ‘for’ loop initial declarations are only allowed in C99 mode
             for (int i=0; parent->sd_enumerate[i]; i++) {
             ^
    ../../../../src/tests/cwrap/../../../src/util/domain_info_utils.c:77:9: note: use option -std=c99 or -std=gnu99 to compile your code
    make[3]: *** [../../../src/util/server_tests-domain_info_utils.o] Error 1
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>