ICU-22457 count() of getUnicodeKeywords is incorrect

The count() is incorrect if the Locale contains extension which is not -u-
for example -x-, -t-.

Currently, this PR only contains tests to show the problem.

ICU-22457 Fix the enum_count

icu4c/source/common/locid.cpp

@@ -2407,8 +2407,9 @@ Locale::getLocaleCache()
 }
 
 class KeywordEnumeration : public StringEnumeration {
-private:
+protected:
     char *keywords;
+private:
     char *current;
     int32_t length;
     UnicodeString currUSKey;
@@ -2515,6 +2516,17 @@ class UnicodeKeywordEnumeration : public KeywordEnumeration {
         if (resultLength != nullptr) *resultLength = 0;
         return nullptr;
     }
+    virtual int32_t count(UErrorCode &/*status*/) const override {
+        char *kw = keywords;
+        int32_t result = 0;
+        while(*kw) {
+            if (uloc_toUnicodeLocaleKey(kw) != nullptr) {
+                result++;
+            }
+            kw += uprv_strlen(kw)+1;
+        }
+        return result;
+    }
 };
 
 // Out-of-line virtual destructor to serve as the "key function".

icu4c/source/test/intltest/loctest.cpp

@@ -3989,6 +3989,8 @@ LocaleTest::TestCreateUnicodeKeywords() {
     LocalPointer<StringEnumeration> keys(l.createUnicodeKeywords(status));
     status.errIfFailureAndReset("\"%s\"", l.getName());
 
+    assertEquals("count", 2, keys->count(status));
+
     const char* key;
     int32_t resultLength;
 
@@ -4139,6 +4141,11 @@ LocaleTest::TestCreateUnicodeKeywordSet() {
                result.find("ca") != result.end());
     assertTrue("set::find(\"co\")",
                result.find("co") != result.end());
+
+    LocalPointer<StringEnumeration> se(l.createUnicodeKeywords(status), status);
+    status.errIfFailureAndReset("\"%s\" createUnicodeKeywords()", l.getName());
+    assertEquals("count()", 2, se->count(status));
+    status.errIfFailureAndReset("\"%s\" count()", l.getName());
 }
 
 void
@@ -4154,6 +4161,10 @@ LocaleTest::TestCreateUnicodeKeywordSetEmpty() {
     status.errIfFailureAndReset("\"%s\"", l.getName());
 
     assertEquals("set::size()", 0, static_cast<int32_t>(result.size()));
+
+    LocalPointer<StringEnumeration> se(l.createUnicodeKeywords(status), status);
+    assertTrue("createUnicodeKeywords", se.isNull());
+    status.expectErrorAndReset(U_MEMORY_ALLOCATION_ERROR);
 }
 
 void
@@ -4174,6 +4185,12 @@ LocaleTest::TestCreateUnicodeKeywordSetWithPrivateUse() {
                result.find("x") == result.end());
     assertTrue("getUnicodeKeywords set::find(\"foo\")",
                result.find("foo") == result.end());
+    assertEquals("set::size()", 1, static_cast<int32_t>(result.size()));
+
+    LocalPointer<StringEnumeration> se(l.createUnicodeKeywords(status), status);
+    status.errIfFailureAndReset("\"%s\" createUnicodeKeywords()", l.getName());
+    assertEquals("count()", 1, se->count(status));
+    status.errIfFailureAndReset("\"%s\" count()", l.getName());
 }
 
 void