tests/docker: allow user to override check target

This is useful when trying to bisect a particular failing test behind
a docker run. For example:

  make docker-test-clang@fedora \
    TARGET_LIST=arm-softmmu \
    TEST_COMMAND="meson test qtest-arm/qos-test" \
    J=9 V=1

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221117172532.538149-4-alex.bennee@linaro.org>

tests/docker/common.rc

@@ -63,12 +63,12 @@ check_qemu()
 {
     # default to make check unless the caller specifies
     if [ $# = 0 ]; then
-        INVOCATION="check"
+        INVOCATION="${TEST_COMMAND:-make $MAKEFLAGS check}"
     else
-        INVOCATION="$@"
+        INVOCATION="make $MAKEFLAGS $@"
     fi
 
-    make $MAKEFLAGS $INVOCATION
+    $INVOCATION
 }
 
 test_fail()