Fix ci_icpc: skip UTF-8 u8-literal comparison test on classic ICC

test-deserialization_cpp20 failed:

  ERROR: CHECK( j2["emoji"] == "😀" ) is NOT correct!

check_utf8() only guards against MSVC's ANSI-codepage quirk (its docstring
example), but classic ICC has an analogous problem: it doesn't encode a
narrow string literal containing non-ASCII source characters as UTF-8,
so comparing a decoded u8R"(...)" literal against a narrow literal with
the same characters fails. Extend the existing guard.

Verified with the pinned astyle 3.4.13; no diff.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-deserialization.cpp

@@ -227,6 +227,11 @@ bool check_utf8()
     // Runtime check of the active ANSI code page
     // 65001 == UTF-8
     return GetACP() == 65001;
+#elif defined(__ICC) || defined(__INTEL_COMPILER)
+    // classic Intel ICC does not encode narrow string literals containing
+    // non-ASCII source characters as UTF-8, so comparing a decoded u8 literal
+    // against a narrow string literal containing the same characters fails
+    return false;
 #else
     return true;
 #endif