t4254: modernize tests

- Don't start tests with 'test $? = 0' to catch preparation done
  outside the test_expect_success block.

- Move writing the bogus patch and the expected output into the
  appropriate test_expect_success blocks.

- Use the test_must_fail helper instead of manually checking for
  non-zero exit code.

- Use the debug-friendly test_path_is_file helper instead of 'test -f'.

- No space after '>'.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4254-am-corrupt.sh

@@ -3,20 +3,19 @@
 test_description='git am with corrupt input'
 . ./test-lib.sh
 
-# Note the missing "+++" line:
-cat > bad-patch.diff <<'EOF'
-From: A U Thor <au.thor@example.com>
-diff --git a/f b/f
-index 7898192..6178079 100644
---- a/f
-@@ -1 +1 @@
--a
-+b
-EOF
-
 test_expect_success setup '
-	test $? = 0 &&
-	echo a > f &&
+	# Note the missing "+++" line:
+	cat >bad-patch.diff <<-\EOF &&
+	From: A U Thor <au.thor@example.com>
+	diff --git a/f b/f
+	index 7898192..6178079 100644
+	--- a/f
+	@@ -1 +1 @@
+	-a
+	+b
+	EOF
+
+	echo a >f &&
 	git add f &&
 	test_tick &&
 	git commit -m initial
@@ -26,17 +25,12 @@ test_expect_success setup '
 #   fatal: unable to write file '(null)' mode 100644: Bad address
 # Also, it had the unwanted side-effect of deleting f.
 test_expect_success 'try to apply corrupted patch' '
-	git am bad-patch.diff 2> actual
-	test $? = 1
+	test_must_fail git am bad-patch.diff 2>actual
 '
 
-cat > expected <<EOF
-fatal: git diff header lacks filename information (line 4)
-EOF
-
 test_expect_success 'compare diagnostic; ensure file is still here' '
-	test $? = 0 &&
-	test -f f &&
+	echo "fatal: git diff header lacks filename information (line 4)" >expected &&
+	test_path_is_file f &&
 	test_cmp expected actual
 '