tests/fish_update_completions: simple system test

tests/checks/fish_update_completions.fish

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+# RUN: %fish %s
+# REQUIRES: command -v sphinx-build
+
+mkdir -p man/man1
+
+echo >man/man1/dirname.1 '
+.SH NAME
+dirname \- strip last component from file name
+.SH DESCRIPTION
+.PP
+Output each NAME with its last non\-slash component and trailing slashes
+.TP
+\fB\-z\fR, \fB\-\-zero\fR
+end each output line with NUL, not newline
+'
+
+MANPATH=$PWD/man fish_update_completions
+# CHECK: Parsing man pages and writing completions to {{.*}}/generated_completions/
+# CHECK: {{.*}}  1 / 1 : dirname.1{{.*}}
+
+cd $__fish_cache_dir
+find -type f
+# CHECK: ./generated_completions/dirname.fish
+
+cat ./generated_completions/dirname.fish
+# CHECK: # dirname
+# CHECK: # Autogenerated from man page {{.*}}/man/man1/dirname.1
+# CHECK: complete -c dirname -s z -l zero -d 'end each output line with NUL, not newline'