[test] add OGS6_msh class

tests/testthat/test-ogs6_msh.R

+test_that("OGS6_msh initialization works", {
+
+    msh_path <- system.file("extdata/benchmarks/t1_1Dsource",
+                            "t1_1Dsource.msh",
+                            package = "r2ogs6")
+
+    msh_obj <- OGS6_msh$new(msh_path = msh_path)
+
+    expect_equal(class(msh_obj)[1], "OGS6_msh")
+    expect_equal(class(msh_obj$msh_path), "character")
+    expect_true(grepl("\\.msh$", msh_obj$msh_path))
+})
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