CTestCustom: fix escape sequence
Fixes noise in the CI build:
CMake Warning (dev) at /builds/gitlab-kitware-sciviz-ci/build/CTestCustom.cmake:51 (set):
Syntax error in cmake code at
/builds/gitlab-kitware-sciviz-ci/build/CTestCustom.cmake:60
when parsing string
plugin/.*Plugin\.h
Invalid escape sequence \.
Policy CMP0010 is not set: Bad variable reference syntax is an error. Run
"cmake --help-policy CMP0010" for policy details. Use the cmake_policy
command to set the policy and suppress this warning.
This warning is for project developers. Use -Wno-dev to suppress it.