Suppress a warning from grep which causes tests to fail.

Taken from upstream pull request:

  https://github.com/aureliojargas/clitest/pull/54

diff --git a/test/inline-match-egrep.sh b/test/inline-match-egrep.sh
--- a/test/inline-match-egrep.sh
+++ b/test/inline-match-egrep.sh
@@ -42,7 +42,7 @@ $ printf ' \t  \t\t   \n'       #=> --egrep ^ 	  		   $
 # egrep regexes. You'll need to test in your system if that's the
 # case. I recommend using a literal tab to avoid problems.
 
-$ printf 'may\tfail'            #=> --egrep ^may\tfail$
+$ printf 'may\tfail'            #=> --egrep ^may\\tfail$
 $ printf 'may\tfail'            #=> --egrep ^may[\t]fail$
 $ printf 'will\tmatch'          #=> --egrep ^will	match$
 
@@ -51,7 +51,7 @@ $ printf 'will\tmatch'          #=> --egrep ^will	match$
 # These tests will fail:
 
 $ printf 'will\nfail'           #=> --egrep will.*fail
-$ printf 'will\nfail'           #=> --egrep will\nfail
+$ printf 'will\nfail'           #=> --egrep will\\nfail
 
 # If one line of a multiline results matches, the test is OK
 
'/guix/'>summary</a><a href='/guix/refs/?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>refs</a><a class='active' href='/guix/log/etc/completion'>log</a><a href='/guix/tree/etc/completion?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>tree</a><a href='/guix/commit/etc/completion?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>commit</a><a href='/guix/diff/etc/completion?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/completion'>
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<tr><td><span title='2021-04-20 12:13:16 +0200'>2021-04-20</span></td><td><a href='/guix/commit/etc/completion?id=d95168321f4a9bf6857b598da0a183b45a868d54'>import: Remove Nix importer.</a><span class='msg-avail'>...<span class='msg-tooltip'>This importer has suffered from bitrot and no longer works with current
Nix and Nixpkgs.  See &lt;https://bugs.gnu.org/32339&gt; and
&lt;https://bugs.gnu.org/36255&gt;.

* guix/import/snix.scm, guix/scripts/import/nix.scm,
tests/snix.scm: Remove.
* Makefile.am (MODULES, SCM_TESTS): Remove them.
* guix/scripts/import.scm (importers): Remove "nix".
* build-aux/test-env.in: Remove NIXPKGS variable.
* configure.ac: Remove '--with-nixpkgs' option.
* doc/guix.texi (Invoking guix import): Remove bit about "guix import
nix".
* etc/completion/fish/guix.fish: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-17 10:52:28 +0100'>2021-02-17</span></td><td><a href='/guix/commit/etc/completion?id=ee2a5da80a9bda25542c00a7a35a9ddddcbd58af'>scripts: system: Remove 'vm-image' command.</a><span class='msg-avail'>...<span class='msg-tooltip'>Remove the 'vm-image' command that has been superseded by the 'image'
command.

* gnu/system/vm.scm (system-qemu-image): Remove it.
* guix/scripts/system.scm (system-derivation-for-action): Mark 'vm-image'
command as deprecated and use the image API to produce the VM image.
(perform-action, show-help): Adapt accordingly.
* tests/guix-system.sh: Ditto.
* doc/guix.texi (Invoking guix system,
Running Guix in a VM): Ditto.
* etc/completion/fish/guix.fish: Ditto.
* etc/completion/zsh/_guix: Ditto.
</span></span></td><td>Mathieu Othacehe</td></tr>