fix OSX/clang unittests

[westram]

[followup from #740]

Under OSX and/or when compiled with clang

• several tests have been disabled
• several tests show different behavior

I've added code-markers to find the relevant code.
perl-grep-expr and current hits listed below:

grepx.pl '' -m 500 -r -A -g "\b((TEST_DISABLED|RESULT_MODIFIED)_(CLANG|OSX))\b" /dev/null
ARBDB/adsocket.cxx:1386:26:        // TEST_DISABLED_OSX: fails for darwin on jenkins (/tmp seems to be a symbolic link there)
ARBDB/arbdb.cxx:2709:22:        // @@@ TEST_DISABLED_OSX: test fails to compile for OSX on build server
ARBDB/gb_aci.cxx:968:51:        // @@@ RESULT_MODIFIED_OSX: this test depends on library version
CORE/arb_cs.cxx:353:18:        // TEST_DISABLED_OSX: this test may fail randomly (always timeouts under OSX)
CORE/arb_diff.cxx:608:22:        // TEST_DISABLED_OSX: section fails compilation under OSX
CORE/arb_mem.cxx:58:22:        // @@@ TEST_DISABLED_CLANG: clang version fails to segfault after allocation failure. needs fix.
CORE/arb_string.cxx:407:18:        // TEST_DISABLED_CLANG: evaluation order differs under clang
SL/REGEXPR/RegExpr.cxx:205:24:        // RESULT_MODIFIED_OSX: regex isn't portable :/

Four locations are more relevant and have additionally been marked with '@@@'.

[westram]

updated results above for r17363