set: Add test cases for setting ENV{VAR} to empty

The behavior differs by platform and if the variable is already set.
Encode existing behavior in tests to preserve it until intentionally
changed.

Issue: #27285

Tests/RunCMake/set/Env-stdout-windows.txt

@@ -0,0 +1,2 @@
+^-- ENV{UNSET_THEN_EMPTY} is not defined
+-- ENV{SET_THEN_EMPTY} is not defined$

Tests/RunCMake/set/Env-stdout.txt

@@ -0,0 +1,2 @@
+^-- ENV{UNSET_THEN_EMPTY} is not defined
+-- ENV{SET_THEN_EMPTY} is defined to ''$

Tests/RunCMake/set/Env.cmake

@@ -0,0 +1,15 @@
+unset(ENV{UNSET_THEN_EMPTY})
+set(ENV{UNSET_THEN_EMPTY} "")
+if(DEFINED ENV{UNSET_THEN_EMPTY})
+  message(STATUS "ENV{UNSET_THEN_EMPTY} is defined to '$ENV{UNSET_THEN_EMPTY}'")
+else()
+  message(STATUS "ENV{UNSET_THEN_EMPTY} is not defined")
+endif()
+
+set(ENV{SET_THEN_EMPTY} "set")
+set(ENV{SET_THEN_EMPTY} "")
+if(DEFINED ENV{SET_THEN_EMPTY})
+  message(STATUS "ENV{SET_THEN_EMPTY} is defined to '$ENV{SET_THEN_EMPTY}'")
+else()
+  message(STATUS "ENV{SET_THEN_EMPTY} is not defined")
+endif()

Tests/RunCMake/set/RunCMakeTest.cmake

@@ -12,3 +12,6 @@ run_cmake(CacheUnknownArguments)
 run_cmake(CacheMissingArguments)
 run_cmake(CacheWrongTYPE)
 run_cmake(CacheSetUnset)
+
+# set(ENV{}) syntax
+run_cmake_script(Env)