Testsuite: Add check for RTS linker
Test static-plugins
requires RTS linker support and in testsuite/tests/plugins/all.T
it is disabled by adding a unless (config.have_ext_interp, skip)
condition. This is confusing.
Introduce another check config.have_RTS_linker
and use that where a test only relies on the RTS linker and not also the external interpreter.