Fix portability issue in tests from commit ce773f230.

Modern POSIX seems to require strtod() to accept "-NaN", but there's
nothing about NaN in SUSv2, and some of our oldest buildfarm members
don't like it.  Let's try writing it as -'NaN' instead; that seems
to produce the same result, at least on Intel hardware.

Per buildfarm.

src/test/regress/expected/hash_func.out

@@ -348,7 +348,7 @@ SELECT hashfloat4('0'::float4) = hashfloat4('-0'::float4) AS t;
  t
 (1 row)
 
-SELECT hashfloat4('NaN'::float4) = hashfloat4('-NaN'::float4) AS t;
+SELECT hashfloat4('NaN'::float4) = hashfloat4(-'NaN'::float4) AS t;
  t 
 ---
  t
@@ -360,7 +360,7 @@ SELECT hashfloat8('0'::float8) = hashfloat8('-0'::float8) AS t;
  t
 (1 row)
 
-SELECT hashfloat8('NaN'::float8) = hashfloat8('-NaN'::float8) AS t;
+SELECT hashfloat8('NaN'::float8) = hashfloat8(-'NaN'::float8) AS t;
  t 
 ---
  t

src/test/regress/sql/hash_func.sql

@@ -258,7 +258,7 @@ DROP TYPE hash_test_t2;
 -- Check special cases for specific data types
 --
 SELECT hashfloat4('0'::float4) = hashfloat4('-0'::float4) AS t;
-SELECT hashfloat4('NaN'::float4) = hashfloat4('-NaN'::float4) AS t;
+SELECT hashfloat4('NaN'::float4) = hashfloat4(-'NaN'::float4) AS t;
 SELECT hashfloat8('0'::float8) = hashfloat8('-0'::float8) AS t;
-SELECT hashfloat8('NaN'::float8) = hashfloat8('-NaN'::float8) AS t;
+SELECT hashfloat8('NaN'::float8) = hashfloat8(-'NaN'::float8) AS t;
 SELECT hashfloat4('NaN'::float4) = hashfloat8('NaN'::float8) AS t;