fix(backend): add missing tests and fix strip code

cmd/karma/tests/testscript/113_receivers_keep_re_invalid.txt

@@ -0,0 +1,14 @@
+karma.bin-should-fail --config.file=karma.yaml
+! stdout .
+cmp stderr stderr.txt
+
+-- stderr.txt --
+level=error msg="Execution failed" error="keep regex rule 'foo.++++' is invalid: error parsing regexp: invalid nested repetition operator: `++`"
+-- karma.yaml --
+alertmanager:
+  servers:
+    - name: default
+      uri: https://127.0.0.1:9093
+receivers:
+  keep_re:
+    - foo.++++

cmd/karma/tests/testscript/114_receivers_strip_re_invalid.txt

@@ -0,0 +1,14 @@
+karma.bin-should-fail --config.file=karma.yaml
+! stdout .
+cmp stderr stderr.txt
+
+-- stderr.txt --
+level=error msg="Execution failed" error="strip regex rule 'foo.++++' is invalid: error parsing regexp: invalid nested repetition operator: `++`"
+-- karma.yaml --
+alertmanager:
+  servers:
+    - name: default
+      uri: https://127.0.0.1:9093
+receivers:
+  strip_re:
+    - foo.++++

internal/config/config.go

@@ -426,7 +426,7 @@ func (config *configSchema) Read(flags *pflag.FlagSet) (string, error) {
 	}
 
 	config.Receivers.CompiledStripRegex = make([]*regexp.Regexp, len(config.Receivers.StripRegex))
-	for i, stripRegex := range config.Receivers.KeepRegex {
+	for i, stripRegex := range config.Receivers.StripRegex {
 		config.Receivers.CompiledStripRegex[i], err = regex.CompileAnchored(stripRegex)
 		if err != nil {
 			return "", fmt.Errorf("strip regex rule '%s' is invalid: %w", stripRegex, err)