Update simplecov from 0.12.0 to 0.21.2 on test (#2556)

This would help get rid of the following errors:

Failure/Error: SimpleCov.start
RuntimeError:
  coverage measurement is already setup

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Co-authored-by: Thomas Reynolds <me@tdreyno.com>

Gemfile.lock

@@ -83,7 +83,7 @@ GEM
       gherkin (~> 4.0)
     cucumber-wire (0.0.1)
     diff-lcs (1.4.4)
-    docile (1.1.5)
+    docile (1.4.0)
     dotenv (2.7.6)
     erubis (2.7.0)
     execjs (2.8.1)
@@ -100,7 +100,6 @@ GEM
     i18n (1.6.0)
       concurrent-ruby (~> 1.0)
     interception (0.5)
-    json (2.5.1)
     kramdown (1.17.0)
     liquid (3.0.6)
     listen (3.0.8)
@@ -184,11 +183,12 @@ GEM
     sassc (2.4.0)
       ffi (~> 1.9)
     servolux (0.13.0)
-    simplecov (0.12.0)
-      docile (~> 1.1.0)
-      json (>= 1.8, < 3)
-      simplecov-html (~> 0.10.0)
-    simplecov-html (0.10.2)
+    simplecov (0.21.2)
+      docile (~> 1.1)
+      simplecov-html (~> 0.11)
+      simplecov_json_formatter (~> 0.1)
+    simplecov-html (0.12.3)
+    simplecov_json_formatter (0.1.4)
     sinatra (2.1.0)
       mustermann (~> 1.0)
       rack (~> 2.2)