Merge branch 'feature/escapable-curly-brackets' into 'master'

ref:quality/dune-testtools Preventing key-dependent resolution. The use case I
am implementing this for is quite expressive:

( ${CMAKE_CXX_COMPILER_ID} STREQUAL Clang ) AND ( {foo} STREQUAL bar ) \|
cmake_guard

Without this commit, meta ini would try to find replace CMAKE_CXX_COMPILER_ID.

See merge request [!120]

  [!120]: gitlab.dune-project.org/quality/dune-testtools/merge_requests/120