unittests: Fix test_compiler_detection on Solaris

Accept Solaris linker in addition to GNU linker.  Previously using the
system provided gcc (which calls the Solaris linker) caused it to fail with:

======================================================================
FAIL: test_compiler_detection (__main__.AllPlatformTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "run_unittests.py", line 2525, in test_compiler_detection
    self.assertIsInstance(cc.linker, mesonbuild.linkers.GnuLikeDynamicLinkerMixin)
AssertionError: <SolarisDynamicLinker: v9.2.0 `gcc`> is not an instance of <class 'mesonbuild.linkers.GnuLikeDynamicLinkerMixin'>

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

run_unittests.py

@@ -2582,6 +2582,8 @@ class AllPlatformTests(BasePlatformTests):
                 self.assertIsInstance(linker, ar)
                 if is_osx():
                     self.assertIsInstance(cc.linker, mesonbuild.linkers.AppleDynamicLinker)
+                elif is_sunos():
+                    self.assertIsInstance(cc.linker, (mesonbuild.linkers.SolarisDynamicLinker, mesonbuild.linkers.GnuLikeDynamicLinkerMixin))
                 else:
                     self.assertIsInstance(cc.linker, mesonbuild.linkers.GnuLikeDynamicLinkerMixin)
             if isinstance(cc, clangcl):