lang: Add per-test config with count maximums

Some of our special tests can only be run once per `go test` invocation.
That is, using the test -count flag will cause a guaranteed failure
since we depend on a global being initialized only once as part of that
test.

This adds a per-test config option so that a user can specify to never
run a particular test more than once. This lets us continue to use the
-count flag with the test suite, without it causing some tests to fail.

lang/interpret_test/TestAstFunc2/test-one-instance-bind.txtar

@@ -1,3 +1,7 @@
+-- CONFIG --
+{
+	"maximum-count": 1
+}
 -- main.mcl --
 import "test"
 # one_instance_a should only produce one value, and will error if initialized twice

lang/interpret_test/TestAstFunc2/test-one-instance-class-arg.txtar

@@ -1,3 +1,7 @@
+-- CONFIG --
+{
+	"maximum-count": 1
+}
 -- main.mcl --
 import "test"
 

lang/interpret_test/TestAstFunc2/test-one-instance-func-arg.txtar

@@ -1,3 +1,7 @@
+-- CONFIG --
+{
+	"maximum-count": 1
+}
 -- main.mcl --
 import "test"
 

lang/interpret_test/TestAstFunc2/test-one-instance-lambda-arg.txtar

@@ -1,3 +1,7 @@
+-- CONFIG --
+{
+	"maximum-count": 1
+}
 -- main.mcl --
 import "test"