Use RUBY_TESTOPTS instead of TESTOPTS

https://github.com/ruby/ruby/pull/2417#issuecomment-526884646
> `TESTOPTS` is defaulted to `$(RUBY_TESTOPTS)` in `common.mk` file.
> Use the latter name to pass options via an environment variable.

.github/workflows/macos.yml

@@ -43,14 +43,14 @@ jobs:
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
-          TESTOPTS: "$JOBS -q --tty=no"
+          RUBY_TESTOPTS: "$JOBS -q --tty=no"
           MSPECOPT: "-ff" # not using `-j` because sometimes `mspec -j` silently dies
         if: matrix.test_task != 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       # test-bundled-gems is separated for marking `continue-on-error` because it randomly fails.
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
-          TESTOPTS: "$JOBS -q --tty=no"
+          RUBY_TESTOPTS: "$JOBS -q --tty=no"
         continue-on-error: true
         if: matrix.test_task == 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       - name: Leaked Globals

.github/workflows/ubuntu.yml

@@ -46,14 +46,14 @@ jobs:
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
-          TESTOPTS: "$JOBS -q --tty=no"
+          RUBY_TESTOPTS: "$JOBS -q --tty=no"
           MSPECOPT: "-ff" # not using `-j` because sometimes `mspec -j` silently dies
         if: matrix.test_task != 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       # test-bundled-gems is separated for marking `continue-on-error` because it randomly fails.
       - name: Tests (test-bundled-gems)
         run: make -s ${{ matrix.test_task }}
         env:
-          TESTOPTS: "-q --tty=no"
+          RUBY_TESTOPTS: "-q --tty=no"
         continue-on-error: true
         if: matrix.test_task == 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       - name: Leaked Globals