BUG: Give the installed itkTestDriver an RPATH to ITK's libraries

[hjmjohnson]

Give the installed itkTestDriver an install RPATH pointing at ITK's library directory, so it can load @rpath/libITKTestKernel (and the rest) from an install prefix without DYLD_LIBRARY_PATH / LD_LIBRARY_PATH.

Symptom and verification

Every external-module test that runs through ITK_TEST_DRIVER against an installed ITK aborts on macOS before running:

dyld: Library not loaded: @rpath/libITKTestKernel-6.0.1.dylib
  Referenced from: <prefix>/bin/itkTestDriver
  Reason: no LC_RPATH's found

ITK installs its libraries with @rpath install names but sets no INSTALL_RPATH anywhere, so the driver has no search path. Found while running remote-module test suites against an installed ITK for #6891.

With this change the installed driver carries LC_RPATH @loader_path/../lib ($ORIGIN/../lib on Linux), and LesionSizingToolkit's five ITK_TEST_DRIVER tests that aborted now pass from the installed prefix with no environment variables set.

Measured on a shared Release build installed to a scratch prefix, configured with -DCMAKE_INSTALL_RPATH=/opt/external/lib to confirm an externally supplied entry survives:

$ otool -l <prefix>/bin/itkTestDriver | grep -A2 LC_RPATH
    path /opt/external/lib
    path @loader_path/../lib
$ env -u DYLD_LIBRARY_PATH <prefix>/bin/itkTestDriver --no-process ; echo $?
0

DYLD_PRINT_LIBRARIES shows all 47 ITK libraries resolving from the prefix with no library-path variable set.

Install layouts

The RPATH is computed from ITK_INSTALL_RUNTIME_DIR and ITK_INSTALL_LIBRARY_DIR, so relative non-default layouts (lib64, a custom bin) resolve correctly.

Both variables are assumed relative to CMAKE_INSTALL_PREFIX, as they already are at CMake/ITKModuleMacros.cmake:533 and :542. An absolute value is rejected at configure time, because by that point the install tree is already wrong. With -DCMAKE_INSTALL_LIBDIR=/usr/lib64 on main today, the exported configuration gets

ITKCommon_RUNTIME_LIBRARY_DIRS = "<build-dir>//usr/lib64"

which is the build directory joined to the absolute path, pointing where nothing is installed; CMake also writes the generated package files into <build-dir>/usr/lib64/. That configuration is broken rather than merely unsupported, so failing early beats shipping an install tree whose exported paths do not exist. Making ITK handle absolute install directories properly spans the module macros and the exported configuration and is tracked separately in #6896.

[hjmjohnson]

Two force-pushes just landed: the first (e26302bb0f4 → b9b00a57974) is the review fixes only, on the original base; the second (b9b00a57974 → dd1e9d70d65) is a plain rebase on main with no content change (git range-diff reports =).

[dzenanz]

LGTM, but my primary dev OS is Windows, so it would good if someone else reviewed this too.