Tests: Fix CheckSourceTree test when build is under source (#15566)

Since the build tree will populate content under the source tree
the test cannot reliably check that the source tree is pristine.
Simply skip most of the test in this case.

Tests/CMakeTests/CheckSourceTreeTest.cmake.in

@@ -16,8 +16,13 @@ string(REPLACE "\\" "\\\\" HOME "${HOME}")
 # (i.e. - is it an "in source" build?)
 #
 set(in_source_build 0)
+set(build_under_source 0)
 
-if(CMake_SOURCE_DIR STREQUAL "${CMake_BINARY_DIR}")
+string(FIND "${CMake_BINARY_DIR}" "${CMake_SOURCE_DIR}/" pos)
+if(pos EQUAL 0)
+  message("build dir is *inside* source dir")
+  set(build_under_source 1)
+elseif(CMake_SOURCE_DIR STREQUAL "${CMake_BINARY_DIR}")
   message("build dir *is* source dir")
   set(in_source_build 1)
 else()
@@ -39,8 +44,13 @@ message("bin_len='${bin_len}'")
 message("substr_len='${substr_len}'")
 message("bin_dir='${bin_dir}'")
 message("in_source_build='${in_source_build}'")
+message("build_under_source='${build_under_source}'")
 message("")
 
+if(build_under_source)
+  message(STATUS "Skipping rest of test because build tree is under source tree")
+  return()
+endif()
 
 # If this does not appear to be a git checkout, just pass the test here
 # and now. (Do not let the test fail if it is run in a tree *exported* from a