Disable parallel testing for 'rust/compile/nr2/compile.exp' [PR119508]

Disable parallel testing for 'rust/compile/nr2/compile.exp' [PR119508]

..., using the standard idiom.  This '*.exp' file doesn't adhere to the
parallel testing protocol as defined in 'gcc/testsuite/lib/gcc-defs.exp'.

This also restores proper behavior for '*.exp' files executing after (!) this
one, which erroneously caused hundreds or even thousands of individual test
cases get duplicated vs. skipped, randomly, depending on the '-jN' level.

	PR testsuite/119508
	gcc/testsuite/
	* rust/compile/nr2/compile.exp: Disable parallel testing.

(cherry picked from commit 79d2c3089f480738613b7d338d86d8be710f8158)