test binaries can't find library in build directory when using RPATH linking

[boegel]

Examples:

• FlexiBLAS (see easybuilders/easybuild-easyblocks@b9bbf02)
• libxc (see libxc regression test fails when using RPATH easybuild-easyconfigs#11900)

We should check if we can come up with a general fix for this, for example by:

• also including $ORIGIN/.. the RPATH section (which would fix the libxc case, but not the FlexiBLAS one);
• not filtering the build directory in the RPATH wrappers during the test step;
• disabling RPATH linking in the test step;