CMake 3.27 deprecates compatibility with CMake < 3.5.  Update tests that
do not cover older interfaces to avoid the deprecation warning.

Follow the pattern from:

* commit 7b07ccdd2b (Tests/*Only: Update cmake_minimum_required versions,
                     2020-06-15, v3.19.0-rc1~629^2~1)

* commit 72e7c45e98 (Tests: Bump CMake minimum required in tests to 2.8.12,
                     2020-12-22, v3.20.0-rc1~224^2)

* commit f6b4db365a (Tests: bump cmake_minimum_required version to 2.8.12,
                     2021-04-04, v3.21.0-rc1~372^2)

Also remove explicit `cmake_policy` settings made redundant by the
version.

This needlessly produces warnings during the test runs that no-one
sees but that are distracting when actually inspecting the logs.

Since commit v3.13.0-rc1~39^2 (add_custom_{command,target}:
WORKING_DIRECTORY generator expressions, 2018-09-22) the
`WORKING_DIRECTORY` option accepts generator expressions.
Fix support for the case of a leading generator expression
by deferring conversion to an absolute path until after
evaluation of the generator expression.

Fixes: #18543

When running an in-source build the CustomCommandWorkingDirectory test
created a copy of a source file in the same directory it was running on.
This breaks when byproducts are cleaned (e.g. via Ninja) because it
deletes one of the source files.

This teaches add_custom_command and add_custom_target WORKING_DIRECTORY
about generator expressions

Fixes: #14089

This was inadvertently testing the assumed sources behavior in Ninja (no
other test seems to exercise it). There is now a test explicitly testing
it in `RunCMake.Ninja`, so fix this test to work properly regardless of
the assumed sources behavior.

Ancient CMake versions required upper-case commands.  Later command
names became case-insensitive.  Now the preferred style is lower-case.

Run the following shell code:

cmake --help-command-list |
grep -v "cmake version" |
while read c; do
    echo 's/\b'"$(echo $c | tr '[:lower:]' '[:upper:]')"'\(\s*\)(/'"$c"'\1(/g'
done >convert.sed &&
git ls-files -z -- bootstrap '*.cmake' '*.cmake.in' '*CMakeLists.txt' |
egrep -z -v '^(Utilities/cm|Source/kwsys/)' |
xargs -0 sed -i -f convert.sed &&
rm convert.sed

Our Git commit hooks disallow modification or addition of lines with
trailing whitespace.  Wipe out all remnants of trailing whitespace
everywhere except third-party code.

Run the following shell code:

git ls-files -z -- \
 bootstrap doxygen.config '*.readme' \
 '*.c' '*.cmake' '*.cpp' '*.cxx' \
 '*.el' '*.f' '*.f90' '*.h' '*.in' '*.in.l' '*.java' \
 '*.mm' '*.pike' '*.py' '*.txt' '*.vim' |
egrep -z -v '^(Utilities/cm|Source/(kwsys|CursesDialog/form)/)' |
egrep -z -v '^(Modules/CPack\..*\.in)' |
xargs -0 sed -i 's/ \+$//'

This also fixes handling of trailing slashes in the directory name.