Bump base OS to Big Sur to get to new Xcode versions

Author: bok-

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   tests-xcode-12_5:
     name: Test Builds - Xcode 12.5 (Beta)
-    runs-on: macOS-latest
+    runs-on: macos-11.0
     env:
       DEVELOPER_DIR: /Applications/Xcode_12.5.app/Contents/Developer
 
@@ -38,7 +38,7 @@ jobs:
 
   tests-xcode-12_4:
     name: Test Builds - Xcode 12.4
-    runs-on: macOS-latest
+    runs-on: macos-11.0
     env:
       DEVELOPER_DIR: /Applications/Xcode_12.4.app/Contents/Developer
 
@@ -61,7 +61,7 @@ jobs:
 
   tests-xcode-12_3:
     name: Test Builds - Xcode 12.3
-    runs-on: macOS-latest
+    runs-on: macos-11.0
     env:
       DEVELOPER_DIR: /Applications/Xcode_12.3.app/Contents/Developer
 
@@ -84,7 +84,7 @@ jobs:
 
   tests-xcode-12_2:
     name: Test Builds - Xcode 12.2
-    runs-on: macOS-latest
+    runs-on: macos-11.0
     env:
       DEVELOPER_DIR: /Applications/Xcode_12.2.app/Contents/Developer
 
@@ -107,7 +107,7 @@ jobs:
 
   tests-xcode-12:
     name: Test Builds - Xcode 12
-    runs-on: macOS-latest
+    runs-on: macos-latest
     env:
       DEVELOPER_DIR: /Applications/Xcode_12.app/Contents/Developer