Skip tests of test if the shell does not implement it

If the testee does not implement the test utility as a built-in, the
test cases in tests/test-p.tst tests the behavior of the external
utility found in $PATH, which may (or may not) affect the results.
We should skip the tests if we are not sure if the shell supports the
built-in.

tests/test-p.tst

@@ -2,6 +2,19 @@
 
 posix="true"
 
+# This file is for testing the shell built-in, so we should skip the tests
+# if the shell does not seem to implement the test built-in.
+case $("$TESTEE" -c 'command -V test') in
+    (*built-in*|*builtin*)
+        # Okay, the testee seems to support the test command as a built-in.
+        # This check is not POSIXly portable because the output of "command -V"
+        # is unspecified in POSIX, but it works for yash and most other shells.
+        ;;
+    (*)
+        skip=true
+        ;;
+esac
+
 umask u=rwx,go=
 
 >file