<format>: Add header unit test (#1816)

Tests format import.

Author: eldakesh-ms

tests/std/tests/P1502R1_standard_library_header_units/custom_format.py

@@ -29,7 +29,7 @@ def getBuildSteps(self, test, litConfig, shared):
             'exception',
             'execution',
             'filesystem',
-            # 'format',
+            'format',
             'forward_list',
             'fstream',
             'functional',

tests/std/tests/P1502R1_standard_library_header_units/custombuild.pl

@@ -28,7 +28,7 @@ ()
         "exception",
         "execution",
         "filesystem",
-        # "format",
+        "format",
         "forward_list",
         "fstream",
         "functional",

tests/std/tests/P1502R1_standard_library_header_units/test.cpp

@@ -29,7 +29,7 @@ import <deque>;
 import <exception>;
 import <execution>;
 import <filesystem>;
-// import <format>;
+import <format>;
 import <forward_list>;
 import <fstream>;
 import <functional>;
@@ -305,7 +305,7 @@ int main() {
 
     {
         puts("Testing <format>.");
-        puts("(TRANSITION, not yet implemented.)");
+        assert(format("{} {}", "testing", "format") == "testing format");
     }
 
     {