ptest-cargo: move run-ptest rc variable initialisation

ptest-cargo run-ptest can be generated in two fashions: generated from
scratch or appended to an exiting run-ptest file. The rc variable used
to track tests failure was only initialized in "generated from scratch"
case. Which lead to errors in the "appended" case.

Move the rc variable initialisation to the common code of both case to
fix this problem.

Only initialize rc if it was not already affected in the recipe provided
run-ptest.

(From OE-Core rev: 177c47b7decd8e2d1e24738d10b0f7db2777a772)

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Cc: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/ptest-cargo.bbclass

@@ -103,10 +103,10 @@ python do_install_ptest_cargo() {
     with open(ptest_script, "a") as f:
         if not script_exists:
             f.write("#!/bin/sh\n")
-            f.write("rc=0\n")                
         else:
             f.write(f"\necho \"\"\n")
             f.write(f"echo \"## starting to run rust tests ##\"\n")               
+        f.write("if [ -z \"$rc\" ]; then rc=0; fi\n")
         for test_path in test_paths:
             script = textwrap.dedent(f"""\
                 if ! {test_path} {rust_test_args}