Now we have conditional IUSE_TESTS.

E.g. autotest-tests has:

X86_IUSE_TESTS="
	+tests_security_SMMLocked
	+tests_xfsFilesystemTestSuite
"

CLIENT_IUSE_TESTS="
	x86? ( ${X86_IUSE_TESTS} )
	amd64? ( ${X86_IUSE_TESTS} )
...
"
...
IUSE_TESTS="${IUSE_TESTS}
	${CLIENT_IUSE_TESTS}
"

IUSE="${IUSE} ${IUSE_TESTS}"

Then, querying use shows:

equery-$BOARD u autotest-tests
[ Legend : U - final flag setting for installation]
[        : I - package is installed with flag     ]
[ Colors : set, unset                             ]
 * Found these USE flags for chromeos-base/autotest-tests-0.0.4-r8562:
 U I
 - - !chromeless_tests?                              : <unknown>
 - - !chromeless_tty?                                : <unknown>
 - - (                                               : <unknown>
 - - )                                               : <unknown>
 - - amd64?                                          : <unknown>
 - - arc-camera3                                     : <unknown>
 - - arc-camera3?                                    : <unknown>
 + - autotest                                        : <unknown>
...


Looks like it does not work as expected.

Assuming there's unconditional tests in IUSE_TESTS, the problematic use in autotest.eclass should be copying files,
https://chromium.git.corp.google.com/chromiumos/overlays/chromiumos-overlay/+/fb968d3353baf835ecf6e71ed110b03fbf9bc4a5/eclass/autotest.eclass#201
and IUSE containing mysterious variables, like '(' etc.

I couldn't figure out how huge the real impact of this bug is, TBH.
At the moment, if the only problematic thing is copying unnecessary files, then, it _looks_ not very much serious, assuming we don't use mysterious USE flags.
If you know other, please speak up :-).