Tests: document try_{compile,run} old/new signature testing

Add a brief documentation blurb to the try_compile and try_run test sets
on writing tests for both "old" and "new" signatures.

Tests/RunCMake/try_compile/old_and_new_signature_tests.cmake

@@ -1,3 +1,12 @@
+# These tests are performed using both the historic and the newer SOURCES
+# signatures of try_compile. It is critical that they behave the same and
+# produce comparable output for both signatures. Tests that cannot do this
+# belong in RunCMakeTests.txt, not here.
+#
+# Tests here MUST include(${CMAKE_CURRENT_SOURCE_DIR}/${try_compile_DEFS}) and
+# use the variables defined therein appropriately. Refer to existing tests for
+# examples.
+
 run_cmake(CopyFileErrorNoCopyFile)
 run_cmake(NoCopyFile)
 run_cmake(NoCopyFile2)

Tests/RunCMake/try_run/old_and_new_signature_tests.cmake

@@ -1,3 +1,12 @@
+# These tests are performed using both the historic and the newer SOURCES
+# signatures of try_run. It is critical that they behave the same and produce
+# comparable output for both signatures. Tests that cannot do this belong in
+# RunCMakeTests.txt, not here.
+#
+# Tests here MUST include(${CMAKE_CURRENT_SOURCE_DIR}/${try_compile_DEFS}) and
+# use the variables defined therein appropriately. Refer to existing tests for
+# examples.
+
 run_cmake(BadLinkLibraries)
 run_cmake(BinDirEmpty)
 run_cmake(BinDirRelative)