testsuite: add jobspecs using string counts

Problem: none of the current valid jobspecs use a resource count string

Add use_case_1.8.yaml from RFC14, resource_count_string_min_only.yaml,
and valid/resource_count_string_range.yaml to the testsuite valid jobspecs

Author: sam-maloney

t/jobspec/valid/resource_count_string_min_only.yaml

@@ -0,0 +1,19 @@
+version: 999
+resources:
+  - type: slot
+    count: "2+"
+    label: default
+    with:
+      - type: node
+        count: 1
+tasks:
+  - command: [ "flux", "start" ]
+    slot: default
+    count:
+      per_slot: 1
+attributes:
+  system:
+    duration: 3600.
+    cwd: "/home/flux"
+    environment:
+      HOME: "/home/flux"

t/jobspec/valid/resource_count_string_range.yaml

@@ -0,0 +1,19 @@
+version: 999
+resources:
+  - type: slot
+    count: "2-32:2:*"
+    label: default
+    with:
+      - type: node
+        count: 1
+tasks:
+  - command: [ "flux", "start" ]
+    slot: default
+    count:
+      per_slot: 1
+attributes:
+  system:
+    duration: 3600.
+    cwd: "/home/flux"
+    environment:
+      HOME: "/home/flux"

t/jobspec/valid/use_case_1.8.yaml

@@ -0,0 +1,19 @@
+version: 999
+resources:
+  - type: slot
+    count: "4,9,16,25"
+    label: default
+    with:
+      - type: node
+        count: 1
+tasks:
+  - command: [ "flux", "start" ]
+    slot: default
+    count:
+      per_slot: 1
+attributes:
+  system:
+    duration: 3600.
+    cwd: "/home/flux"
+    environment:
+      HOME: "/home/flux"