vcl: fix Unicode cast in mnemonic test

Fix the following:

/home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/vcl/qa/cppunit/mnemonic.cxx:35:50:error: in LIBO_INTERNAL_ONLY code, replace literal cast to 'sal_Unicode' (aka 'char16_t') with a u'...' char16_t character literal [loplugin:salunicodeliteral]
    CPPUNIT_ASSERT_EQUAL(sResult[1], sal_Unicode('~'));
                                     ~~~~~~~~~~~~^~~~
/home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/workdir/UnpackedTarball/cppunit/include/cppunit/TestAssert.h:300:32: note: expanded from macro 'CPPUNIT_ASSERT_EQUAL'
                              (actual),                \
                               ^~~~~~
1 error generated.

Change-Id: I323270d4ddf9e8ae22f9bf7cd7333a8d7e58d17c
Reviewed-on: https://gerrit.libreoffice.org/40301Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>

vcl/qa/cppunit/mnemonic.cxx

@@ -32,7 +32,7 @@ void VclMnemonicTest::testMnemonic()
 {
     MnemonicGenerator aGenerator;
     OUString sResult = aGenerator.CreateMnemonic(OUString::fromUtf8(u8"ßa"));
-    CPPUNIT_ASSERT_EQUAL(sResult[1], sal_Unicode('~'));
+    CPPUNIT_ASSERT_EQUAL(sResult[1], u'~');
 }
 
 CPPUNIT_TEST_SUITE_REGISTRATION(VclMnemonicTest);