So that they use the guile (or guile.exe, in the MinGW case) that was
just built, and not just any guile that can be found in the PATH.
* test-suite/standalone/test-asmobs: Use ${GUILE:-guile} instead of
guile.
* test-suite/standalone/test-bad-identifiers: Ditto.
* test-suite/standalone/test-fast-slot-ref.in: Ditto.
* test-suite/standalone/test-require-extension: Ditto.
* test-suite/standalone/test-system-cmds: Ditto.
* test-suite/standalone/test-use-srfi.in: Ditto.