ci: support multi-value xdebug.mode

Upstream fixed a bug with parsing comma-separated INI values, but
requires quoting the value. This commit adds that necessary
quoting.

@see shivammathur/setup-php#392

.github/workflows/ci-2.x.yml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       matrix:
         # All the versions, OS, and dependency levels we want to support
-        php: [7.4, 7.3, 7.2, 7.1]
+        php: [7.4, 7.3, 7.2, 7.1, 7.0]
         dependency: [stable] # TODO: lowest
         os: [ubuntu]         # TODO: windows, macos
         # In XDebug 2 and earlier, if XDebug extension was present, the function
@@ -45,8 +45,9 @@ jobs:
         # has paths for with- and without- XDebug, and we want to test both of them.
         # However, we require XDebug for code coverage, so before XDebug 3 there was
         # no way to test the without-XDebug code path. Now we can, so we do.
+        # Note: INI values with embedded commas need to have their value quoted.
         # @see https://xdebug.org/docs/all_settings#mode
-        xdebug3-mode: ["xdebug.mode=develop,coverage", "xdebug.mode=coverage"]
+        xdebug3-mode: ["xdebug.mode='develop,coverage'", "xdebug.mode=coverage"]
         include:
           # 7.4 introduced an engine wide flag that disables arguments in
           # backtraces. The default in 7.4 is On. However, we have tests that