support/testing/test_rust: use standard defconfig fragment style

support/testing/test_rust: use standard defconfig fragment style

Since commit "2927f412be support/testing: standardize defconfig
fragments style" all other test cases use the same style for defconfig
fragments:
 - start after a backslash;
 - be declared as a multi-line string literal;
 - be indented one level more than the variable that contains it.

Do the same here for consistency.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>