The number of exponents is too damn high (quantumlib#5456)

Just going to throw this out there for comment.

`assist_consistent_protocols` is used in a huge number of tests (many of which are themselves parameterized tests).  We test all of the protocols for a large number of different `pow`ed versions of the gate.  This ends up being something like 10% or our test time.  

I've reduced this number down to what I think is a bit more reasonable, (0, itself, inverse, a non-zero float, a symbol).

Author: dabacon

cirq/testing/consistent_protocols.py

@@ -42,7 +42,7 @@
 def assert_implements_consistent_protocols(
     val: Any,
     *,
-    exponents: Sequence[Any] = (0, 1, -1, 0.25, -0.5, 0.1, sympy.Symbol('s')),
+    exponents: Sequence[Any] = (0, 1, -1, 0.25, sympy.Symbol('s')),
     qubit_count: Optional[int] = None,
     ignoring_global_phase: bool = False,
     setup_code: str = 'import cirq\nimport numpy as np\nimport sympy',
@@ -79,7 +79,7 @@ def assert_implements_consistent_protocols(
 def assert_eigengate_implements_consistent_protocols(
     eigen_gate_type: Type[ops.EigenGate],
     *,
-    exponents: Sequence[value.TParamVal] = (0, 0.5, 1, -1, 0.25, -0.5, 0.1, sympy.Symbol('s')),
+    exponents: Sequence[value.TParamVal] = (0, 1, -1, 0.25, sympy.Symbol('s')),
     global_shifts: Sequence[float] = (0, -0.5, 0.1),
     qubit_count: Optional[int] = None,
     ignoring_global_phase: bool = False,