feat(fail): support valid patterns for Fail matcher (#14)

This change adds support for validating that the error caught by the
Fail matcher will match one of a set of regex patterns. This enables
using the "Fail" matcher to ensure that not only failure in general, but
specific failure specifically.

Author: arikkfir

matchers_fail.go

@@ -1,9 +1,12 @@
 package justest
 
-import "reflect"
+import (
+	"reflect"
+	"regexp"
+)
 
 //go:noinline
-func Fail() Matcher {
+func Fail(patterns ...string) Matcher {
 	return MatcherFunc(func(t T, actuals ...any) {
 		GetHelper(t).Helper()
 
@@ -19,8 +22,17 @@ func Fail() Matcher {
 
 		lastRT := reflect.TypeOf(last)
 		if lastRT.AssignableTo(reflect.TypeOf((*error)(nil)).Elem()) {
-			// ok, no-op
-			return
+			if len(patterns) > 0 {
+				for _, pattern := range patterns {
+					re := regexp.MustCompile(pattern)
+					if re.MatchString(last.(error).Error()) {
+						return
+					}
+				}
+				t.Fatalf("Error message did not match any of these patterns: %v", patterns)
+			} else {
+				return
+			}
 		}
 
 		t.Fatalf("No error occurred")

matchers_fail_test.go

@@ -2,6 +2,7 @@ package justest_test
 
 import (
 	"fmt"
+	"regexp"
 	"testing"
 
 	. "github.com/arikkfir/justest"
@@ -40,4 +41,16 @@ func TestFail(t *testing.T) {
 			With(mt).Verify(tc.actuals...).Will(Fail()).OrFail()
 		})
 	}
+	t.Run("Succeeds if error matches one of the patterns", func(t *testing.T) {
+		t.Parallel()
+		mt := NewMockT(t)
+		defer mt.Verify(SuccessVerifier())
+		With(mt).Verify(fmt.Errorf("expected error")).Will(Fail(`^expected error$`)).OrFail()
+	})
+	t.Run("Fails if error matches none of the patterns", func(t *testing.T) {
+		t.Parallel()
+		mt := NewMockT(t)
+		defer mt.Verify(FailureVerifier(regexp.QuoteMeta(`Error message did not match any of these patterns: [^abc$ ^def$ ^ghi$]`)))
+		With(mt).Verify(fmt.Errorf("expected error")).Will(Fail(`^abc$`, `^def$`, `^ghi$`)).OrFail()
+	})
 }