tests/meson.build: unknown variable softhsm

When building with meson and at least one of hwtests dependencies is not found, meson complains at configure time with message like in the title, about the hwtests block which has depends: [softhsm]. I don't know meson build system yet, but at least one possible solution is to move the block starting at "if pkcs11_tool_dep.found().." at the end and put hwtests block within the if block.