We have but ELF binutils tests in the MIPS subset, and non-ELF MIPS/GAS
ports are gone, making a future addition of any non-ELF tests unlikely.
Bail out right away then if non-ELF, consuming one level of indentation
across the actual tests run.
binutils/
* testsuite/binutils-all/mips/mips.exp: Bail out right away if
non-ELF.