Fix spurious errors in SAM test files

Ensure field names are distinct. Negate F1 values so they differ from
the F0 values. In aux.fail-f1.sam's F0 and F1, use values smaller than
the minimum subnormal value representable so they underflow to 0 when
evaluated as float. Fixes #722.

Author: jmarshall

test/sam/failed/aux.fail-f1.sam

@@ -1,3 +1,3 @@
 @CO	Validation of AUX
 @CO	Type f
-I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:1.275494351E-38	f0:f:1.275494351E-38	F1:f:3.502823466E+38	F1:f:-3.502823466E+38
+I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:1E-46	F1:f:-1E-46	F2:f:3.502823466E+38	F3:f:-3.502823466E+38

test/sam/passed/aux.pass-f.sam

@@ -4,4 +4,4 @@ I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:-1	F1:f:0	F2:f:1	F3:f:9.9e-19	F4:f:-9.9E-19	F5:f:
 I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:0	F1:f:-0	F2:f:+0
 I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:09	F1:f:-009e+0	F2:f:+00009e-0
 I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:00.1	F1:f:.1	F2:f:-00.1	F3:f:-.1
-I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:1.175494351E-38	f0:f:1.175494351E-38	F1:f:3.402823466E+38	F1:f:-3.402823466E+38
+I	4	*	0	0	*	*	0	0	CAT	QQQ	F0:f:1.175494351E-38	F1:f:-1.175494351E-38	F2:f:3.402823466E+38	F3:f:-3.402823466E+38