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

@@ -3,8 +3,9 @@
 test_description='git am with corrupt input'
 . ./test-lib.sh
 
+test_expect_success setup '
 	# Note the missing "+++" line:
-cat > bad-patch.diff <<'EOF'
+	cat >bad-patch.diff <<-\EOF &&
 	From: A U Thor <au.thor@example.com>
 	diff --git a/f b/f
 	index 7898192..6178079 100644
@@ -14,8 +15,6 @@ index 7898192..6178079 100644
 	+b
 	EOF
 
-test_expect_success setup '
-	test $? = 0 &&
 	echo a >f &&
 	git add f &&
 	test_tick &&
@@ -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_must_fail git am bad-patch.diff 2>actual
-	test $? = 1
 '
 
-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 &&
+	echo "fatal: git diff header lacks filename information (line 4)" >expected &&
-	test -f f &&
+	test_path_is_file f &&
 	test_cmp expected actual
 '