Support WSL detection (#61)

`uname -s` was already being executed every run for *cygwin* detection.
I've consolidated all of the OS detection into a single function. This
also fixed the problem of running `uname -s` twice for the `alt`
command.

test/006_unit_set_operating_system.bats

@@ -0,0 +1,76 @@
+load common
+load_fixtures
+
+@test "Default OS" {
+  echo "
+    By default, the value of OPERATING_SYSTEM should be reported by uname -s
+  "
+
+  # shellcheck source=/dev/null
+  YADM_TEST=1 source "$T_YADM"
+  status=0
+  output=$( set_operating_system; echo "$OPERATING_SYSTEM" ) || {
+    status=$?
+    true
+  }
+
+  expected=$(uname -s 2>/dev/null)
+
+  echo "output=$output"
+  echo "expect=$expected"
+
+  [ "$status" == 0 ]
+  [ "$output" = "$expected" ]
+}
+
+@test "Detect no WSL" {
+  echo "
+    When /proc/version does not contain Microsoft, report uname -s
+  "
+
+  echo "proc version exists" > "$BATS_TMPDIR/proc_version"
+
+  # shellcheck source=/dev/null
+  YADM_TEST=1 source "$T_YADM"
+  # shellcheck disable=SC2034
+  PROC_VERSION="$BATS_TMPDIR/proc_version"
+  status=0
+  output=$( set_operating_system; echo "$OPERATING_SYSTEM" ) || {
+    status=$?
+    true
+  }
+
+  expected=$(uname -s 2>/dev/null)
+
+  echo "output=$output"
+  echo "expect=$expected"
+
+  [ "$status" == 0 ]
+  [ "$output" = "$expected" ]
+}
+
+@test "Detect WSL" {
+  echo "
+    When /proc/version contains Microsoft, report WSL
+  "
+
+  echo "proc version contains Microsoft in it" > "$BATS_TMPDIR/proc_version"
+
+  # shellcheck source=/dev/null
+  YADM_TEST=1 source "$T_YADM"
+  # shellcheck disable=SC2034
+  PROC_VERSION="$BATS_TMPDIR/proc_version"
+  status=0
+  output=$( set_operating_system; echo "$OPERATING_SYSTEM" ) || {
+    status=$?
+    true
+  }
+
+  expected="WSL"
+
+  echo "output=$output"
+  echo "expect=$expected"
+
+  [ "$status" == 0 ]
+  [ "$output" = "$expected" ]
+}

test/113_accept_jinja_alt.bats

@@ -94,7 +94,7 @@ function test_alt() {
   # shellcheck source=/dev/null
   YADM_TEST=1 source "$T_YADM"
   process_global_args -Y "$T_DIR_YADM"
-  test_cygwin
+  set_operating_system
   configure_paths
 
   status=0