Tests: Add unit tests for property redefinition

From reading `Source/cmDefinePropertyCommand.cxx` and
`Source/cmPropertyDefinition.cxx`, attempts to use `define_property()`
to redefine an existing property will be silently ignored.
Once a property is defined, it cannot be redefined.

Add some unit tests to confirm this behavior of `define_property()`
remains as expected based on the current implementation.

Tests/RunCMake/define_property/RunCMakeTest.cmake

@@ -1,6 +1,7 @@
 include(RunCMake)
 
 run_cmake(define_property)
+run_cmake(define_property-redefine)
 run_cmake(define_property-INITIALIZE_FROM_VARIABLE)
 run_cmake(define_property-INITIALIZE_FROM_VARIABLE-invalid_1)
 run_cmake(define_property-INITIALIZE_FROM_VARIABLE-invalid_2)

Tests/RunCMake/define_property/define_property-redefine.cmake

@@ -0,0 +1,88 @@
+function(verify_value type prop attrib expected actual)
+  if(expected STREQUAL "FALSE")
+    if(actual)
+      message(FATAL_ERROR
+        "Expected ${type} property ${prop}'s ${attrib} to be false")
+    endif()
+  elseif(expected STREQUAL "TRUE")
+    if(NOT actual)
+      message(FATAL_ERROR
+        "Expected ${type} property ${prop}'s ${attrib} to be true")
+    endif()
+  elseif(NOT actual STREQUAL expected)
+    message(FATAL_ERROR
+      "Expected value of ${type} property ${prop}'s ${attrib}:\n"
+      "  ${expected}\n"
+      "Actual value:\n"
+      "  ${actual}"
+    )
+  endif()
+endfunction()
+
+function(assert_tgt_prop_attrib_eq prop attrib expected)
+  get_property(actual TARGET NONE PROPERTY "${prop}" "${attrib}")
+  verify_value(TARGET "${prop}" "${attrib}" "${expected}" "${actual}")
+endfunction()
+
+function(assert_dir_prop_attrib_eq prop attrib expected)
+  get_property(actual DIRECTORY "" PROPERTY "${prop}" "${attrib}")
+  verify_value(DIRECTORY "${prop}" "${attrib}" "${expected}" "${actual}")
+endfunction()
+
+#
+# TESTS
+#
+
+# Define a new target property
+message(CHECK_START "Testing define_property(TARGET ...)")
+define_property(TARGET PROPERTY TGT1
+  BRIEF_DOCS "Brief")
+assert_tgt_prop_attrib_eq(TGT1 BRIEF_DOCS "Brief")
+assert_tgt_prop_attrib_eq(TGT1 FULL_DOCS "NOTFOUND")
+message(CHECK_PASS "Complete")
+
+# Attempt to redefine with different/additional attributes
+message(CHECK_START "Testing TARGET property redefinition")
+define_property(TARGET PROPERTY TGT1
+  BRIEF_DOCS "Changed"
+  FULL_DOCS "Full")
+assert_tgt_prop_attrib_eq(TGT1 BRIEF_DOCS "Brief")
+assert_tgt_prop_attrib_eq(TGT1 FULL_DOCS "NOTFOUND")
+message(CHECK_PASS "Complete")
+
+# Query undefined property
+message(CHECK_START "Testing undefined TARGET property query")
+assert_tgt_prop_attrib_eq(TGT2 DEFINED FALSE)
+assert_tgt_prop_attrib_eq(TGT2 BRIEF_DOCS "NOTFOUND")
+message(CHECK_PASS "Complete")
+
+# Define after query
+message(CHECK_START "Testing TARGET property definition after query")
+define_property(TARGET PROPERTY TGT2
+  BRIEF_DOCS "Brief"
+  FULL_DOCS "Full"
+)
+assert_tgt_prop_attrib_eq(TGT2 DEFINED TRUE)
+assert_tgt_prop_attrib_eq(TGT2 BRIEF_DOCS "Brief")
+assert_tgt_prop_attrib_eq(TGT2 FULL_DOCS "Full")
+message(CHECK_PASS "Complete")
+
+# Define a new directory property
+message(CHECK_START "Testing define_property(DIRECTORY ...)")
+define_property(DIRECTORY PROPERTY DIR1
+  BRIEF_DOCS "Brief"
+  FULL_DOCS "Full"
+)
+assert_dir_prop_attrib_eq(DIR1 DEFINED TRUE)
+assert_dir_prop_attrib_eq(DIR1 BRIEF_DOCS "Brief")
+assert_dir_prop_attrib_eq(DIR1 FULL_DOCS "Full")
+message(CHECK_PASS "Complete")
+
+# Attempt to redefine existing attributes
+message(CHECK_START "Testing DIRECTORY property redefinition")
+define_property(DIRECTORY PROPERTY DIR1
+  BRIEF_DOCS "Overwritten"
+)
+assert_dir_prop_attrib_eq(DIR1 BRIEF_DOCS "Brief")
+assert_dir_prop_attrib_eq(DIR1 FULL_DOCS "Full")
+message(CHECK_PASS "Complete")