Tests: Factor out RunCMake ctest environment isolation

Some RunCMake tests unset some environment variables that affect ctest's
behavior before running it.  Any test using the RunCTest helper needs to
do this, so do it in the helper itself.

Issue: #24153

Tests/RunCMake/CTestCommandLine/RunCMakeTest.cmake

@@ -3,9 +3,6 @@ include(RunCTest)
 
 set(RunCMake_TEST_TIMEOUT 60)
 
-unset(ENV{CTEST_PARALLEL_LEVEL})
-unset(ENV{CTEST_OUTPUT_ON_FAILURE})
-
 run_cmake_command(repeat-opt-bad1
   ${CMAKE_CTEST_COMMAND} --repeat until-pass
   )

Tests/RunCMake/RunCTest.cmake

@@ -1,5 +1,9 @@
 include(RunCMake)
 
+# Isolate our ctest runs from external environment.
+unset(ENV{CTEST_PARALLEL_LEVEL})
+unset(ENV{CTEST_OUTPUT_ON_FAILURE})
+
 function(run_ctest CASE_NAME)
   configure_file(${RunCMake_SOURCE_DIR}/test.cmake.in
                  ${RunCMake_BINARY_DIR}/${CASE_NAME}/test.cmake @ONLY)

Tests/RunCMake/ctest_environment/RunCMakeTest.cmake

@@ -1,9 +1,5 @@
 include(RunCTest)
 
-# Isolate our ctest runs from external environment.
-unset(ENV{CTEST_PARALLEL_LEVEL})
-unset(ENV{CTEST_OUTPUT_ON_FAILURE})
-
 set(CASE_SOURCE_DIR "${RunCMake_SOURCE_DIR}")
 set(RunCTest_VERBOSE_FLAG "-VV")
 

Tests/RunCMake/ctest_fixtures/RunCMakeTest.cmake

@@ -1,9 +1,5 @@
 include(RunCTest)
 
-# Isolate our ctest runs from external environment.
-unset(ENV{CTEST_PARALLEL_LEVEL})
-unset(ENV{CTEST_OUTPUT_ON_FAILURE})
-
 function(run_ctest_test CASE_NAME)
   set(CASE_CTEST_FIXTURES_ARGS "${ARGN}")
   run_ctest(${CASE_NAME})

Tests/RunCMake/ctest_memcheck/RunCMakeTest.cmake

@@ -4,8 +4,6 @@ set(SITE test-site)
 set(BUILDNAME test-build)
 set(COVERAGE_COMMAND "")
 
-unset(ENV{CTEST_PARALLEL_LEVEL})
-
 function(run_mc_test CASE_NAME CHECKER_COMMAND)
   run_ctest(${CASE_NAME} ${ARGN})
 endfunction()

Tests/RunCMake/ctest_test/RunCMakeTest.cmake

@@ -1,9 +1,6 @@
 include(RunCTest)
 set(RunCMake_TEST_TIMEOUT 60)
 
-unset(ENV{CTEST_PARALLEL_LEVEL})
-unset(ENV{CTEST_OUTPUT_ON_FAILURE})
-
 set(CASE_CTEST_TEST_ARGS "")
 set(CASE_CTEST_TEST_LOAD "")