feat: basic_fixed_string implementation and testing improved

test/runtime/fmt_test.cpp

@@ -34,7 +34,6 @@
 #ifdef MP_UNITS_MODULES
 import mp_units;
 #else
-#include <mp-units/ext/fixed_string.h>
 #include <mp-units/format.h>
 #include <mp-units/ostream.h>  // IWYU pragma: keep
 #include <mp-units/systems/cgs.h>
@@ -51,18 +50,6 @@ inline constexpr bool mp_units::is_vector<T> = true;
 using namespace mp_units;
 using namespace mp_units::si::unit_symbols;
 
-TEST_CASE("fixed_string", "[text][ostream][fmt]")
-{
-  basic_fixed_string txt = "units";
-  SECTION("iostream")
-  {
-    std::ostringstream os;
-    os << txt;
-    CHECK(os.str() == "units");
-  }
-  SECTION("fmt") { CHECK(MP_UNITS_STD_FMT::format("{}", txt) == "units"); }
-}
-
 TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
 {
   std::ostringstream os;