selftest/multiconfig: Test that multiconfigs in separate layers works

We should test that mutliconfigs from a layer work, not just build/conf.
This adds such a test.

[YOCTO #13566]

(From OE-Core rev: 2306261fb85d5d03145989c3af9c6897111644ae)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/conf/multiconfig/muslmc.conf

@@ -0,0 +1,2 @@
+TCLIBC = "musl"
+TMPDIR = "${TOPDIR}/tmp-mc-musl"

meta/lib/oeqa/selftest/cases/multiconfig.py

@@ -70,3 +70,16 @@ TMPDIR = "${TOPDIR}/tmp-mc-tiny"
 
         result = bitbake('mc:test:multiconfig-test-parse -c showvar')
         self.assertIn('MCTESTVAR=test2', result.output.splitlines())
+
+    def test_multiconfig_inlayer(self):
+        """
+        Test that a multiconfig from meta-selftest works.
+        """
+
+        config = """
+BBMULTICONFIG = "muslmc"
+"""
+        self.write_config(config)
+
+        # Build a core-image-minimal, only dry run needed to check config is present
+        bitbake('mc:muslmc:bash -n')