SMTChecker: Bring back counterexample checks in regression tests

Since the default is now to ignore the counterexamples when checking
test output, we bring back counterexample checks in tests where the
counterexample is (mostly) deterministic.

test/libsolidity/smtCheckerTests/external_calls/call_mutex_unsafe.sol

@@ -21,7 +21,7 @@ contract C {
 }
 // ====
 // SMTEngine: all
-// SMTIgnoreCex: yes
+// SMTIgnoreCex: no
 // ----
 // Warning 9302: (212-228): Return value of low-level calls not used.
-// Warning 6328: (232-246): CHC: Assertion violation happens here.
+// Warning 6328: (232-246): CHC: Assertion violation happens here.\nCounterexample:\nx = 0, lock = false\n_a = 0x0\ny = 1\n\nTransaction trace:\nC.constructor()\nState: x = 0, lock = false\nC.set(1)\nState: x = 1, lock = false\nC.f(0x0)\n    _a.call("aaaaa") -- untrusted external call, synthesized as:\n        C.set(0) -- reentrant call

test/libsolidity/smtCheckerTests/external_calls/external_reentrancy_2.sol

@@ -12,5 +12,6 @@ contract C {
 }
 // ====
 // SMTEngine: all
+// SMTIgnoreCex: no
 // ----
-// Warning 6328: (117-131): CHC: Assertion violation happens here.
+// Warning 6328: (117-131): CHC: Assertion violation happens here.\nCounterexample:\nlocked = false\ntarget = 0x0\n\nTransaction trace:\nC.constructor()\nState: locked = true\nC.call(0x0)\n    D(target).e() -- untrusted external call, synthesized as:\n        C.call(0x0) -- reentrant call

test/libsolidity/smtCheckerTests/external_calls/underflow_only_in_external_call.sol

@@ -18,6 +18,7 @@ contract C {
 }
 // ====
 // SMTEngine: chc
+// SMTIgnoreCex: no
 // SMTTargets: underflow
 // ----
-// Warning 3944: (109-112): CHC: Underflow (resulting value less than 0) happens here.
+// Warning 3944: (109-112): CHC: Underflow (resulting value less than 0) happens here.\nCounterexample:\nv = 0, guard = false\n = 0\n\nTransaction trace:\nC.constructor()\nState: v = 0, guard = true\nC.f()\n    C.dec() -- trusted external call