test-lib: rearrange start/end of test_expect_* and test_skip

This moves

* the early setup part from test_skip to a new function test_start_

* the final common parts of test_expect_* to a new function
  test_finish_

to make the next commit more obvious.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 5251009..ad6d60a 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -343,6 +343,7 @@
 }
 
 test_expect_failure () {
+	test_start_
 	test "$#" = 3 && { test_prereq=$1; shift; } || test_prereq=
 	test "$#" = 2 ||
 	error "bug in the test script: not 2 or 3 parameters to test-expect-failure"
@@ -357,10 +358,11 @@
 			test_known_broken_failure_ "$1"
 		fi
 	fi
-	echo >&3 ""
+	test_finish_
 }
 
 test_expect_success () {
+	test_start_
 	test "$#" = 3 && { test_prereq=$1; shift; } || test_prereq=
 	test "$#" = 2 ||
 	error "bug in the test script: not 2 or 3 parameters to test-expect-success"
@@ -375,7 +377,7 @@
 			test_failure_ "$@"
 		fi
 	fi
-	echo >&3 ""
+	test_finish_
 }
 
 # test_external runs external test scripts that provide continuous