Tests: Fix failures when running under the default install prefix (#15566)

Fix test cases whose behavior differs when their source or build tree is
under CMAKE_INSTALL_PREFIX by setting an install prefix under the build
tree.  Otherwise they may fail when run under the default install prefix
(e.g. /usr/local).

Tests/FindPackageTest/CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required (VERSION 2.6)
 project(FindPackageTest)
 
+# Protect tests from running inside the default install prefix.
+set(CMAKE_INSTALL_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/NotDefaultPrefix")
+
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR})
 
 # Look for a package which uses FindPackageHandleStandardArgs.cmake with the

Tests/RunCMake/CMP0041/RunCMakeTest.cmake

@@ -1,5 +1,8 @@
 include(RunCMake)
 
+# Protect tests from running inside the default install prefix.
+set(RunCMake_TEST_OPTIONS "-DCMAKE_INSTALL_PREFIX=${RunCMake_BINARY_DIR}/NotDefaultPrefix")
+
 run_cmake(CMP0041-OLD)
 run_cmake(CMP0041-NEW)
 run_cmake(CMP0041-WARN)

Tests/RunCMake/IfacePaths/RunCMakeTest.cmake

@@ -6,6 +6,9 @@ macro(run_cmake test)
   _run_cmake(${test})
 endmacro()
 
+# Protect tests from running inside the default install prefix.
+set(RunCMake_TEST_OPTIONS "-DCMAKE_INSTALL_PREFIX=${RunCMake_BINARY_DIR}/NotDefaultPrefix")
+
 run_cmake(RelativePathInInterface)
 run_cmake(RelativePathInGenex)
 run_cmake(export-NOWARN)

Tests/RunCMake/include_directories/RunCMakeTest.cmake

@@ -1,5 +1,8 @@
 include(RunCMake)
 
+# Protect tests from running inside the default install prefix.
+set(RunCMake_TEST_OPTIONS "-DCMAKE_INSTALL_PREFIX=${RunCMake_BINARY_DIR}/NotDefaultPrefix")
+
 run_cmake(NotFoundContent)
 run_cmake(DebugIncludes)
 run_cmake(TID-bad-target)