merge the C code's ci.yml into the top level one

CI is not currently working due to quota issues, so I'll need to check
that this is actually working later.

.github/workflows/ci.yml

@@ -62,3 +62,13 @@ jobs:
       if: startsWith(matrix.arch, 'armv7-') || startsWith(matrix.arch, 'aarch64-')
     # Test vectors. Note that this uses a hacky script due to path dependency limitations.
     - run: ./test_vectors/cross_test.sh --target ${{ matrix.arch }}
+
+  # Currently only on x86.
+  c_tests:
+    name: C tests
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - run: make test
+      working-directory: ./c

c/.github/workflows/ci.yml

@@ -1,12 +0,0 @@
-name: tests
-
-on: [push]
-
-jobs:
-  x86_tests:
-    name: x86 tests
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v1
-    - run: make test