A simple test for join(1) based on tjr's test case for -o 0.

Partially based on:	PR standards/36072

tools/regression/usr.bin/Makefile

@@ -1,5 +1,5 @@
 # $FreeBSD$
 
-SUBDIR=	file2c uudecode uuencode xargs
+SUBDIR=	file2c join uudecode uuencode xargs
 
 .include <bsd.subdir.mk>

tools/regression/usr.bin/join/Makefile

@@ -0,0 +1,4 @@
+# $FreeBSD$
+
+all:
+	@sh ${.CURDIR}/regress.sh ${.CURDIR}

tools/regression/usr.bin/join/regress.1.in

@@ -0,0 +1,4 @@
+!Name,Data1
+Foo,1
+Bar,2
+Baz,3

tools/regression/usr.bin/join/regress.2.in

@@ -0,0 +1,4 @@
+!Name,Data2
+Foo,Bar
+Baz,2
+Foobar,1

tools/regression/usr.bin/join/regress.out

@@ -0,0 +1,5 @@
+!Name,Data1,Data2
+Foo,1,Bar
+Bar,2,(unknown)
+Baz,3,2
+Foobar,(unknown),1

tools/regression/usr.bin/join/regress.sh

@@ -0,0 +1,17 @@
+# $FreeBSD$
+
+# Go into the regression test directory, handed to us by make(1)
+TESTDIR=$1
+if [ -z "$TESTDIR" ]; then
+  TESTDIR=.
+fi
+cd $TESTDIR
+
+join -t , -a1 -a2 -e '(unknown)' -o 0,1.2,2.2 regress.1.in regress.2.in | \
+  diff -u regress.out -
+if [ $? -eq 0 ]; then
+  echo "Test detected no regression, output matches."
+else
+  echo "Test failed: regression detected.  See above."
+  exit 1
+fi