[test-engine-extras] don't create splits of opposing amt/value signs

Splits' value and amount should never be of opposite signs. Fix
test book which had created this incorrect value/amount
pair. Strengthen env-create-multisplit-transaction to disallow
test book errors.

bindings/guile/test/test-engine-extras.scm

@@ -223,6 +223,9 @@
    #:memo memo
    #:notes notes))
 
+(define (opposite-signs? a b)
+  (< (* a b) 0))
+
 ;; creates multisplit transaction.
 ;;
 ;; input: DD/MM/YY - posting date of transaction
@@ -262,6 +265,9 @@
                   (action (and (> (vector-length split) 3)
                                (vector-ref split 3)))
                   (newsplit (xaccMallocSplit book)))
+              (when (opposite-signs? val amt)
+                (error "env-create-multisplit-transaction error for" DD '/ MM '/ YY
+                       'opposing-signs: 'val val 'amt amt))
               (xaccSplitSetParent newsplit txn)
               (xaccSplitSetAccount newsplit acc)
               (xaccSplitSetValue newsplit val)

gnucash/report/reports/standard/test/test-ifrs-cost-basis.scm

@@ -139,7 +139,7 @@
 
   (env-create-multisplit-transaction
    env 16 04 2020
-   (list (vector usd-cash 2500 -2500)
+   (list (vector usd-cash 2500 2500)
          (vector spy -2500 0 "Buy")
          )
    #:description "Return of Capital"

gnucash/report/test/test-commodity-utils.scm

@@ -167,7 +167,7 @@
                           #:description "Buy MSFT 1500") ;;1500 @ $28.11
     (env-transfer-foreign env 20 01 2012 checking-dem dmlr-a 1500 80
                           #:description "Buy DMLR 80") ;;80 @ DM1500.00
-    (env-transfer-foreign env 20 02 2012 checking-dem dmlr-a -1610 80
+    (env-transfer-foreign env 20 02 2012 checking-dem dmlr-a -1610 -80
                           #:description "Sell DMLR 80") ;;80 @ DM1610.00
     (env-transfer-foreign env 20 02 2012 capgain-dem dmlr-a 110 0
                           #:description "DMLR 80 G/L") ;;80 @ DM1610.00