Fix parallel build in hdrd/tests

hdrl_multiiter-test and hdrl_frameiter-test use the same base names for their data with a pid suffix but remove all files without using the pid suffix :

char * fn = cpl_sprintf("frameiter%zu_%zd.fits", i, (intptr_t)getpid()); ... if (system("rm -f frameiter*fits")) {}

So potentially, in parallel builds, tests can remove the files from the other one which happens systematically to me, which leads to a sigsev. This fix removes only the pid suffixed .fits created by the current process.