t4210: skip i18n tests that don't work on FreeBSD

A number of t4210-log-i18n tests added in 4e2443b181 set LC_ALL to a UTF-8
locale (is_IS.UTF-8) but then pass an invalid UTF-8 string to --grep.
FreeBSD's regcomp() fails in this case with REG_ILLSEQ, "illegal byte
sequence," which git then passes to die():

fatal: command line: '�': illegal byte sequence

When these tests were added the commit message stated:

| It's possible that this
| test breaks the "basic" and "extended" backends on some systems that
| are more anal than glibc about the encoding of locale issues with
| POSIX functions that I can remember

which seems to be the case here.

Extend test-lib.sh to add a REGEX_ILLSEQ prereq, set it on FreeBSD, and
add !REGEX_ILLSEQ to the two affected tests.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4210-log-i18n.sh

@@ -70,7 +70,7 @@ do
 	then
 	    force_regex=.*
 	fi
-	test_expect_success !MINGW,GETTEXT_LOCALE,$prereq "-c grep.patternType=$engine log --grep does not find non-reencoded values (latin1 + locale)" "
+	test_expect_success !MINGW,!REGEX_ILLSEQ,GETTEXT_LOCALE,$prereq "-c grep.patternType=$engine log --grep does not find non-reencoded values (latin1 + locale)" "
 		cat >expect <<-\EOF &&
 		latin1
 		utf8
@@ -84,7 +84,7 @@ do
 		test_must_be_empty actual
 	"
 
-	test_expect_success !MINGW,GETTEXT_LOCALE,$prereq "-c grep.patternType=$engine log --grep does not die on invalid UTF-8 value (latin1 + locale + invalid needle)" "
+	test_expect_success !MINGW,!REGEX_ILLSEQ,GETTEXT_LOCALE,$prereq "-c grep.patternType=$engine log --grep does not die on invalid UTF-8 value (latin1 + locale + invalid needle)" "
 		LC_ALL=\"$is_IS_locale\" git -c grep.patternType=$engine log --encoding=ISO-8859-1 --format=%s --grep=\"$force_regex$invalid_e\" >actual &&
 		test_must_be_empty actual
 	"

t/test-lib.sh

@@ -1422,7 +1422,7 @@ else
 	'
 fi
 
-# Fix some commands on Windows
+# Fix some commands on Windows, and other OS-specific things
 uname_s=$(uname -s)
 case $uname_s in
 *MINGW*)
@@ -1453,6 +1453,12 @@ case $uname_s in
 	test_set_prereq SED_STRIPS_CR
 	test_set_prereq GREP_STRIPS_CR
 	;;
+FreeBSD)
+	test_set_prereq REGEX_ILLSEQ
+	test_set_prereq POSIXPERM
+	test_set_prereq BSLASHPSPEC
+	test_set_prereq EXECKEEPSPID
+	;;
 *)
 	test_set_prereq POSIXPERM
 	test_set_prereq BSLASHPSPEC