Fix tests for out-of-source builds using CMake

[doganulus]

Newer CMake versions allow out-of-source builds using -S and -B options in configuration (CMake 3.13 or newer).

cmake -S ${PROJECT_SOURCE_DIR} -B ${PROJECT_BUILD_DIR}
cmake --build ${PROJECT_BUILD_DIR}
ctest --test-dir ${PROJECT_BUILD_DIR} --output-on-failure
cmake --install ${PROJECT_BUILD_DIR}

Currently, some tests rely on relative paths assuming a build directory ./build such as:

• libeconf/tests/tst-econftool_cat.sh

  Line 16 in 25eb424

  econftool_exe="$PWD/../util/econftool"

• libeconf/tests/tst-econftool_show1.sh

  Line 16 in 25eb424

  econftool_exe="$PWD/../util/econftool"

This makes these tests fail when built out-of-source.