tests: accommodate testing alternate target-triple

For example:

    make build-static-i686
    make -C t STG_TARGET=i686-unknown-linux-musl STG_PROFILE=for-pkg

Author: jpgrayson

t/Makefile

@@ -7,6 +7,7 @@
 # STG_PROVE_OPTS = --jobs=9
 STG_PROVE_OPTS ?=
 STG_PROFILE ?=
+STG_TARGET ?=
 SHELL_PATH ?= $(SHELL)
 TEST_SHELL_PATH ?= $(SHELL_PATH)
 PERL_PATH ?= /usr/bin/perl

t/test-bin/stg

@@ -6,4 +6,9 @@ then
 else
     profile_dir="$STG_PROFILE"
 fi
-exec "$STG_ROOT/target/$profile_dir/stg" "$@"
+if [ -n "$STG_TARGET" ]
+then
+    exec "$STG_ROOT/target/$STG_TARGET/$profile_dir/stg" "$@"
+else
+    exec "$STG_ROOT/target/$profile_dir/stg" "$@"
+fi